POLYIMIDES FOR ADVANCED APPLICATIONS
by
David C. Rich
S.B., Chemical Engineering
Massachusetts Institute of Technology, 1992
Submitted to the Department of Materials Science and Engineering
in Partial Fulfillment of the Requirements for the Degree of
DOCTOR OF PHILOSOPHY
in Polymers
at the
Massachusetts Institute of Technology
May 3, 1996
@Massachusetts Institute of Technology. All rights reserved.
Signature of Author.....................................
.......
David Rich
Department of Materials Science and Engineering
Certified bv............................
Professor Peggy Cebe
-m,--4i Qipervisor
Accepted by......
................................................
rilc
r-
.............
ui iviiael Rubner
Chairman, Departmental Committee on Graduate Studies
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OF TECHNOLOGY
JUN 2 41996 Science
UBRPARiES
POLYIMIDES FOR ADVANCED APPLICATIONS
by
DAVID C. RICH
Submitted to the Department of Materials Science and Engineering
on May 3, 1996 in partial fulfillment of the requirements for the
Degree of Doctor of Philosophy in Polymers
Thesis Supervisor: Professor Peggy Cebe
ABSTRACT
The processing, structure, and properties of aromatic polyimides are investigated
for their use as liquid crystal alignment layers and in other advanced applications.
Probimide 32 polyamide-imide and Probimide 412 inherently photosensitive polyimide are
the primary materials which are evaluated. The physical and chemical changes that occur in
the polyimide films as the materials undergo cloth brushing, UV irradiation, and thermal
curing treatments are of chief interest. Polyimide films are studied by conventional
characterization techniques, by property prediction methods, and by a new technique called
"alignment layer relaxation," which can be used to detect thermal transitions in polymer
surface layers. The alignment relaxation technique is based upon the observation that as
alignment layers are cured above their glass transition temperatures, their liquid crystal
aligning properties are irreversibly destroyed. The aligning performance of polymer films
is evaluated by the phase-sensitive laser optical transmission of parallel-aligned nematic
liquid crystal cells placed between crossed polarizers. A non-contact method of alignment
of surface aligning liquid crystals is also introduced, in which polyamide-imide alignment
films are irradiated with linearly polarized short wavelength ultraviolet light (,=254 nm).
The technique produces alignment that appears to be comparable with that achieved by
conventional contact brushing. The photo-aligned polyamide-imide is believed to develop
its aligning properties via a chain scission process that occurs in the alignment films as a
consequence of the UV exposure, though the exact mechanism of the alignment is not
completely understood at this time. The removal of solvent from polyimide films upon
3
curing is investigated by thermogravimetric analysis and Fourier transform infrared
spectroscopy. Solvent is found to be rapidly eliminated from polyamide-imide thin films
(-450A) at low temperatures (-110 0C). High temperature cures are found to chemically
crosslink the polyimides, especially if the treatment proceeds in an air environment. Also,
comparison of predicted and measured refractive indices of polyimides materials indicate
that, upon thermal imidization, non-fluorinated polyimides undergo chain packing and/or
charge transfer processes that appear to substantially mitigated in polyimides with bulky
fluorinated spacer groups.
Table of Contents
Abstract .........................................................................................................................
2
Table of Contents...................................................................................................
List of Figures........................................................................................................
List of Tables.................................................................................................................
Acknowledgments .........................................................................................................
4
9
19
21
Chapter 1. Introduction and Theory..................................................................
22
1.1. Synthesis and Processing............................
1.1.1. General scheme.......................................................................................
1.1.2. Photosensitive polyimide precursors.......................................................
1.1.3. Preimidized polyimides.............................................
1.1.4. Preimidized, photosensitive polyimides.............................
........
22
22
24
24
26
1.2. Structure ...............................................................................................................
1.2.1. Macroscopic orientation of polymer films...............................................
1.2.2. Crystalline packing .................................................
1.2.3. Chain-chain interactions................................................................
1.2.4. Liquid crystalline nematic order and alignment..............................
26
26
26
28
29
1.3. Properties.............................................................................................................
1.3.1. Quantitative relationship between polymer
chemistry and refractive index.................................................................
1.3.2. Property changes associated with charge
transfer complexation.............................................
1.3.3. Anisotropy of optical properties..............................................................
1.3.4. Measurement of index of refraction...................
..........
1.3.5. Optical transmission model for liquid crystal alignment..........................
30
30
37
39
45
46
1.4. Performance: Liquid Crystal Alignment Layers...............................
....... 53
1.4.1. Liquid crystal surface alignment on brushed
polymer films..........................................................................................
53
1.4.2. Alternative treatments to polymer layers for liquid
crystal alinnment......
..........................
cr
st
l
ale
m
n
.
..
....
...............
...........................
1.4.3. An LCD device: the TN-LCD............................
...
............... 58
1.5. References............................................................................................................
60
Chapter 2. Refractive Indices of Aromatic Polyimides.................................
.......
72
2.1. Introduction .........................................................................................................
72
2.2. Experimental Techniques.................................................
2.2. 1. Polyimide film preparation.......................................
2.2.2. Zone drawing ..........................................................................................
2.2.3. Refractive index measurement .........................
..................................
2.2.4. Refractive index prediction......................................................................
74
74
74
75
76
2.3. Relationship Between Polyimide Chemistry and Refractive Index.......................
2.3.1. Amorphous, non-fluorinated polyimides............................
.......
2.3.2. Semicrystalline polyimides ..........................................
2.3.3. Polyimides containing hexafluoroisopropylidene (6F) ............................
2.3.4. Polyimides containing fluorinated alkoxy side chains .............................
2.3.5. Polyimides containing pentafluorosulfanyl (SF5)...................................
76
76
77
77
78
79
2.4. Relationship Between Polyimide Processing and Refractive Index...................
2.4.1. In-plane birefringence in polyimide films.................................. ......
2.4.2. Birefringence in zone drawn films..................................
.........
2.4.3. Effect of cure temperature on refractive index.........................................
2.4.4. Effect of post-cure annealing on NEW-TPI polyimide ............................
89
89
90
91
96
2.5. Conclusions..........................................................................................................
96
2.6. References............................................................................................................
99
Chapter 3. Curing Study of Probimide® 412 Preimidized
Photosensitive Polyimide...........................................................................
104
3.1. Introduction .........................................................................
...........................
104
3.2. Experimental........................................................................................................
3.2.1. Polymer film samples.................................
3.2.2. Measurement techniques...... ......................
105
105
106
3.3. Results and Discussion...............................
3.3.1. Coloration and brittleness .............................
3.3.2. Thermogravimetric analysis..............................
3.3.3. Dynamic mechanical analysis..................................................................
3.3.4. Index of refraction.............................
3.3.5. Fourier transform infrared spectroscopy .................................................
108
108
108
116
124
124
3.4. Conclusions ..........................................................................................................
128
3.5. References......................
.............................................................................. 139
Chapter 4. Effect of Cure Conditions on Probimide® 32
Polyamide-imide........................................................................................
141
4. 1. Introduction..........................................................................................................
141
4.2. Experimental.........................................................................................................
4.2.1. Polymer film processing.....................................................................
4.2.2. Measurement techniques ..............................
142
142
143
4.3. Results and Discussion..............................
4.3.1. Coloration and brittleness .............................
4.3.2. Thermogravimetric analysis ....................................................................
4.3.3. Dynamic mechanical analysis..................................................................
4.3.4. Index of refraction.................................................. ............................
4.3.5. Fourier transform infrared spectroscopy............................
143
143
144
152
153
160
4.3.6. Differential scanning calorimetry............................................................. 167
4.4. Conclusions ..........................................................................................................
185
4.5. References............................................................................................................
185
Chapter 5. Alignment of Liquid Crystals on Brushed
Polymer Layers..................................................................................
187
5.1. Introduction..........................................................................................................
187
5.2. Experimental Section.......................................................
5.2.1. Construction of LC alignment cells .........................................................
.................
5.2.2. Evaluation of alignment cells...................................
188
189
195
5.3. Results and Discussion............................
5.3.1. Brushing-induced alignment in
Probimide 32 polyamide-imide...............................................................
5.3.2. Brushing-induced alignment in
other polymers.................................
5.3.3. FTIR spectra of Probimide 32
polyamide-imide alignment films................................
5.3.4. Atomic force microscopy of brushed
polyamide-imide alignment films............................
5.3.5. In situ thermal stability of LC alignment on
brushed Probimide 32 polyamide-imide layers................. .................
5.3.6. "Liquid crystal alignment relaxation" - a new
technique for measuring thermal transitions
in surface layers............................
205
5.4. Experimental Error....................................
227
5.5. Conclusions ..........................................................................................................
228
5.6. References ...........................................................................................................
229
205
207
209
210
216
217
Chapter 6. Liquid Crystal Surface Alignment on Photoirradiated Polymer Layers..............................................
232
6.1. Introduction.......................................................................................................... 232
6.2. Experimental Section.......................................................
6.2.1. Photo-alignment technique............................
..................
6.2.2. Comments regarding the methodology by which the
PAI was selected....................................................
232
232
6.3. Results and Discussion....................................................
6.3.1. Evidence of liquid crystal alignment upon LP-UV exposure.................
6.3.2. Effect of unpolarized UV exposure.............................
..............
6.3.3. Effects of linearly polarized UV exposure................................................
234
234
237
240
6.4. Conclusions..........................................................................................................
250
6.5. Subjects for further work.....................................................................................
250
6.6. References ......................................................................................................
252
Appendix A. Description of Nematic Alignment in Terms of the
Poincare Sphere ..................................
253
A. 1. The Poincare sphere...................................
253
A.2. Changes in polarization state of the Poincare sphere...............................
254
A.3. Examples of nematic LC orientation...............................
257
A.4. Effect of misalignment of the LCs.............
259
...................
A.5. References ......................................................................................................
234
259
List of Figures
Figure 1.1.
General synthesis of polyimides............................................................ 23
Figure 1.2.
Processing scheme of photosensitive polyimide precursors............... 25
Figure 1.3.
Types of orientation in polymer films. a.) isotropic orientation,
b.) in-plane orientation, and c.) uniaxial orientation..............................
Figure 1.4.
Charge transfer interactions in polyimides...........................
Figure 1.5.
A nematic liquid crystalline phase with director D' ...............................
Figure 1.6.
Representations of nematic liquid crystalline material.
a.) no alignment and b.) bulk alignment along director D'..................
Figure 1.7.
Sample calculation of the index of refraction of polystyrene
using the van Krevelen / Vogel technique.........................
Figure 1.8.
Bicerano prediction for the refractive index of polystyrene................ 38
Figure 1.9.
Examples of optical anisotropy. a.)birefringence on the
molecular level, b.) birefringence on the microscopic level
in a polymer spherulite, c.) birefringence on the microscopic
level in a nematic grain, d.) bulk birefringence in an in-plane
oriented film, and e.) bulk birefringence in a film of aligned
liquid crystals..................................................
41
A thick semicrystalline polymer block that is microscopically
birefringent but has nx=ny=nz................................................................
44
Figure 1.10.
...... 28
29
Figure 1.11.
Prism coupling technique for measuring index of refraction............... 45
Figure 1.12.
Graphical representation of a parallel-aligned liquid crystal
plate between a polarizer (p) and an analyzer (A)............................... 46
Figure 1.13.
Plot of I(0)/I(450) vs ...................................................
48
Figure 1.14.
Elliptically polarized light with handedness (left or right),
azimuth (a), and ellipticity (13= a/b).................................
49
.......
Figure 1.15.
Parallel-aligned liquid crystals with inhomogeneities..............
50
Figure 1.16.
Rays of light passing through different points 1 or 2 of
the inhomogenous liquid crystal sample and emerging at 1'
or 2' with different polarization states................................
51
......
Figure 1.17.
Alignment of liquid crystals on brushed polymer surfaces................. 53
Figure 1.18.
A polymer surface being rubbed by a brushing wheel........................... 54
Figure 1.19.
The photochemical reaction in polyvinylmethoxycinnamate
and the oriented structure, proposed by Schadt, that is formed
by exposure to linearly polarized UV........................................
56
Anisotropic chain degradation proposed to occur in a polyimide
film during exposure to linearly polarized UV .......................................
57
States of a twisted nematic liquid crystal display (TN-LCD)
device, a.) off state and b.) on state.................................
59
Figure 1.20.
Figure 1.21.
.......
Figure 2.1.
Zone drawing method..................................................................
75
Figure 2.2.
Experimental and predicted refractive indices of non-fluorinated
polyimides in Table 2.1.................................. .....
.............. 79
Figure 2.3.
Wide angle X-ray scattering (WAXS) of: a.)PMDA-ODA and
b.) ODPA-PDA......................................................
Figure 2.4.
Measured and predicted refractive indices of semicrystalline
polyimides in Table 2.2.........................................................................
82
Figure 2.5.
Measured and predicted refractive indices of 6F polyimides
in Table 2.3.........................................................
Figure 2.6.
Predicted refractive indices (Bicerano's method) of polyimides
with fluorinated alkoxy side chains.....................................................
Figure 2.7.
Refractive index vs. draw ratio for 6FDA-4BDAF in the direction
of the draw and normal to the draw.............................
Figure 2.8.
Refractive indices of several polyimides as a function of
cure temperature............................................
Figure 2.9.
Predicted (Bicerano's method) vs. measured refractive indices
of several polyimides. a.) Predicted values of the polyamic acid
precursors vs. measured values of the films after only 100 0C
cure and b.) Predicted values of the polyimides vs. measured
..................... 94
values after 3000 C cure................................
Figure 2.10.
a.) Average refractive index of NEW-TPI vs. post-cure
annealing temperature and b.) Birefringence vs. post-cure
annealing temperature ............................................................................
97
Figure 2.11.
a.) Average refractive index vs. post-cure annealing time
at 2900 C and b.) Birefringence vs. post-cure annealing time
at 2900C........................................................... 98
Figure 3.1.
Preparation of bulk films. a.) Coating and blading process for
optimization of film smoothness and thickness uniformity.
b.) Pre-cure air-drying chamber..............................
107
Thermogravimetric weight loss and derivative of weight loss vs.
temperature for Probimide® 412 thick films with highest cure
temperature: (a) room temperature, (b) 100 0C, (c) 140 0 C,
(d) 2000 C, and (e) 300 0C................................
110
Figure 3.2.
Figure 3.3.
Figure 3.4.
Figure 3.5.
Figure 3.6.
Thermogravimetric weight loss vs. temperature for
Probimide® 412 thick films baked at 100 0C, UV irradiated or
unirradiated .....................................................................................
...... 115
1 Hz dynamic Young's modulus and loss factor, tanS, vs.
temperature for Probimide® 412 thick films with highest cure
temperature: (a) 100 0C, (b) 2000C, (c) 300 0C (in nitrogen),
(d) 400 0C (in nitrogen) ...........
...........................
118
1 Hz dynamic Young's modulus and loss factor, tans, vs.
temperature for Probimide® 412 thick films cured at 100 0 C,
UV irradiated (dashed line) or unirradiated (solid line) ..........................
122
1 Hz dynamic Young's modulus and loss factor, tan6, vs.
temperature for Probimide® 412 thick films cured at 300 0C,
in air (dashed line) or in nitrogen (solid line).....................................
123
Figure 3.7.
Indices of refraction of spin coated Probimide® 412 films as
a function of highest cure temperature..........................
............. 125
Figure 3.8.
Indices of refraction of spin coated Probimide 412 films as
a function of UV exposure time.................................
...............
Figure 3.9.
126
Relative IR absorbance vs. wavenumber for spin coated
Probimide® 412 films softbaked 15 minutes at 100 0C and
exposed to UV: (a) 0 minutes (no irradiation), (b) 2 minutes,
(c) 10 minutes, (d) 50 minutes........................................... .......... 130
Figure 3.10. Relative IR absorbance vs. wavenumber (2000-4000 cm -1
range) for spin coated Probimide® 412 films with highest
cure temperature treatment: (a) 300 0C (3 hours in nitrogen),
(b) 3000 C (1 hour in air), (c) 3000 C (3 hours in air),
(d) 4000C (1 hour in nitrogen)..............
...........................
134
Figure 3.11.
Figure 4.1.
Figure 4.2.
Relative IR absorbance vs. wavenumber (400-2000 cm -1
range) for spin coated Probimide® 412 films with highest
cure temperature treatment: (a) 3000C (3 hours in nitrogen),
(b) 3000 C (1 hour in air), (c) 3000 C (3 hours in air),
.......
(d) 4000 C (1 hour in nitrogen).............. ..........
135
Weight loss and derivative of weight loss vs. temperature
for N-methyl pyrrolidone (NMP)...............................
145
Weight loss and derivative of weight loss vs. temperature for
Probimide® 32 thick films: a.) dried under an air current at
room temperature, b.) then baked 6 hours at 100 0 C, c.) then
3 hours at 2000C, d.) then 3 hours at 300 0C......................................
146
Figure 4.3.
Weight loss and derivative of weight loss vs. temperature for
Probimide® 32 thick films baked at 100 0 C for 1, 6, or 24 hours.......... 151
Figure 4.4.
Young's modulus and dissipation factor, tan5, for Probimide®
32 thick films: a.) cured 6 hours at 100 0C, b.) then 3 hours at
2000 C, c.) then 3 hours at 3000 C in either air or, d.) nitrogen............ 154
Figure 4.5.
a.) Indices of refraction of spin coated Probimide® 32 films vs.
highest cure temperature in air. Cure cycle was 15 minutes at
100 0C, 1 hour at 200 0C, 1 hour at 3000 C. b.) Refractive
indices for different cure conditions at 300 0C................................
158
Relative absorbance vs. wavenumber (400-2000 cm-1) for
spin coated Probimide® 32 films: a.) baked 10 min. at 100 0C,
b.) 10 min. at 120 0C, c.) 10 min. at 140 0C, d.) 10 min. at 160 0 C,
e.) 10 min. at 180 0C, f.) 10 min. at 2000C ............................................
168
Relative absorbance vs. wavenumber (2000-4000 cm- 1) for
spin coated Probimide® 32 films: a.) baked 10 minutes at 100 0C,
b.) then 10 minutes at 120 0C, c.) then 10 minutes at 140 0C,
d.) then 10 minutes at 160 0C, e.) then 10 minutes at 180 0C,
f.) then 10 minutes at 200 0C................................
174
Figure 4.6.
Figure 4.7.
Figure 4.8.
Figure 4.9.
Figure 4.10.
Relative absorbance vs. wavenumber (400-2000 cm- 1) for
spin coated Probimide® 32 films: a.) baked 15 minutes at 100 0C,
b.) then 1 hour at 2000 C, c.) then 1 hour at 300 0 C in either
air or, d.) nitrogen ................................
175
Relative absorbance vs. wavenumber (2000-4000 cm- 1)
for spin coated Probimide® 32 films: a.) baked 15 minutes at
100 0 C, b.) then 1 hour at 2000 C, c.) then 1 hour at 3000 C in either
air or, d.) nitrogen ................................
179
Ratio of relative absorbance of peak at 1680 cm- 1 to that of the
1370 cm -1 vs. highest cure temperature..........................
180
Figure 4.11.
Endothermic heat flow vs. temperature for Probimide® 32 thick
films: a.) baked 6 hours at 100 0C, b.) then 3 hours at 2000 C,
c.) then 3 hours at 3000 C in either air or, d.) nitrogen........................ 181
Figure 5.1.
Alignment film brushing technique............................
191
Figure 5.2.
Cell constuction procedure. (a.) alignment film formation,
(b.) empty cell construction, (c.) cell filling, (d.) cell annealing,
and (e.) cell sealing ...................................................
193
Qualitative observations of liquid crystal alignment cells
between crossed polarizers.....................................................................
196
Figure 5.3.
Figure 5.4.
Optical transmission apparatus with phase-sensitive detection........... 198
Figure 5.5.
Instrument connections for phase-sensitve detection.......................... 199
Figure 5.6.
Measure relative intensity vs. expected relative intensity for
phase-sensitive detection of neutral density filtered light.
a.) linear scale, and b.) log-log scale................................
201
Figure 5.7.
Measure relative intensity vs. expected relative intensity for
phase-sensitive detection of light diminished by two
consecutive polarizers with their polarizing axes lying at
an angle X. a.) linear scale, and b.) log-log scale.............................. 202
Figure 5.8.
Adjustment made for a cell with small degree of twist
distortion q. The first liquid crystal surface director N is
oriented parallel to polarizer P. The second surface director
is N'. The analyzer is adjusted by q from A to A' such that
A' is perpendicular to N'......................................
204
Figure 5.9.
a.) Intensity / intensity at 450 vs. rotation angle for a nematic
liquid crystal cell aligned with brushed Probimide 32 polyamide
-imide layers. The x-axis is the azimuth of the alignment
director with respect to the polarizer. b.) Relative intensity
vs. rotation angle for nematic liquid crystal cell with untreated
Probimide 32 polyamide-imide layers. The x-axis represents
the azimuth of an arbitrary reference axis with respect to the
polarizer................................................................................................. 206
Figure 5.10.
Values of 4, the effective angle of misalignment, for liquid
crystal cells produced with alignment films of brushed
PMDA-APB polyimide (PI), PMDA-APB poly(amic
acid) (PAA), polyamide-imide (PAI), poly(phenylene
ether sulfone) (PPES), poly(vinyl alcohol) (PVA),
poly(methyl methacrylate) (PMMA), and polycarbonate (PC).
All cells but those produced with PMMA and PC appear to have
very strong macroscopic alignment .............................
.
209
Relative absorbance vs. wavenumber for Probimide 32
polyamide-imide coated from 3wt% solids content and
softbaked at 110C, with final film thickness approximately
450A ......................................................................................................
211
Figure 5.11.
Figure 5.12.
Figure 5.13.
Relative absorbance vs. wavenumber for a brushed Probimide
32 polyamide-imide thin film. a.) Incident infrared electric
field polarized parallel to the brushing direction.
b.) Incident infrared electric field polarized perpendicular
to the brushing direction ..............................
212
Atomic force microscopy (AFM) images of polyamideimide films. a.) brushed, and b.) unbrushed........................................ 214
Figure 5.14. 1(00)/1(450) vs. temperature for a polyamide-imide aligned
liquid crystal cell undergoing an annealing cycle of
systematically increasing temperature treatments................................... 216
Figure 5.15. 1(00)/1(450) vs. post-brush cure temperature for LC alignment cells
produced with Probimide 32 polyamide-imide alignment films.......... 218
Figure 5.16.
Depiction of the structure of the alignment layer films during
alignment relaxation process of the polyamide-imide. The
amorphous chains begin randomly coiled in the plane of the
film (a). They are oriented by the brushing process (b). The
orientation is reversed by annealing above Tg (c)............................... 219
Figure 5.17.
1(00)/1(450) vs. post-brush cure temperature for liquid crystal
Figure 5.18.
Figure 5.19.
alignment cells produced with brushed polyamide-imide that
were either softbaked (filled circles) or cured at 300 0C in air
(open circles) prior to brushing.............................
220
1(00)/1(450) vs. post-brush cure temperature for liquid crystal
alignment cells produced with brushed poly(phenylene ether
sulfone) alignment films................................
221
1(00)/1(450) vs. post-brush cure temperature for liquid crystal
alignment cells produced with brushed PMDA-APB alignment
films. a.) PMDA-APB poly(amic acid) softbaked only,
b.) PMDA-APB hard baked 275 0C 1 hour, and c.) PMDA-APB
cured at 221 0 C for 30 minutes..............................
223
Figure 5.20.
Figure 6.1.
Figure 6.2.
Figure 6.3.
vs. post-brush cure temperature for liquid crystal
alignment cells produced with poly(vinyl alcohol) alignment
layers...................................
226
Exposure technique for irradiating alignment films with linearly
polarized ultraviolet light (LP-UV)............................
233
I(0)/I(450) vs. rotation angle for liquid crystal alignment cell
produced with polyamide-imide alignment layers that have
been irradiated with LP-UV for 25 minutes..............................
235
1(00)/1(450)
1(00)/1(450) vs. LP-UV exposure time for liquid crystal cells
produced with LP-UV irradiated polyamide-imide alignment
films.......................................................... 236
Figure 6.4.
Figure 6.5.
Relative absorbance vs. wavenumber for a Probimide 32
polyamide-imide film. a.) unexposed to UV, and b.) UV
exposed.................................................................................................
238
1(00)/1(450) vs. post-brush curing temperature for
liquid crystal cells produced with UV exposed (squares)
and unexposed (circles) polyamide-imide alignment layers................ 240
Figure 6.6.
Relative absorbance vs. wavenumber for polyamide-imide
layers exposed to linearly polarized ultraviolet (.=245 nm).
a.) 2 hour exposure, UV axis parallel to incident IR axis,
b.) 2 hour exposure, UV axis perpendicular to incident IR axis,
c.) 4 hour exposure, UV axis parallel to incident IR axis, and
d.) 4 hour exposure, UV axis perpendicular to incident IR axis............ 242
Figure 6.7
I(00)/1(450) vs. post-aligning cure temperature for liquid
crystals cells produced with polyamide-imide alignment films
that have been exposed to LP-UV (triangles) or brushed (circles)......... 246
Figure 6.8.
Atomic force microscopy (AFM) image of LP-UV irradiated
polyamide-imide films...........................................................................
247
Figure 6.9.
1(0')/1(450) vs. post-aligning unpolarized UV exposure time
for liquid crystal cells produced from polyamide-imide
alignment films that were exposed to LP-UV (open circles)
or brushed (filled circles) ..............................
249
Figure A.I.
Lines of latitude (1)on the Poincare sphere representing
ellipticity ([3)........... ........................................................................ 253
Figure A.2.
Lines of longitude (L) on the Poincare sphere representing
azimuth (a) .....................................
254
Movement along the Poincare sphere from reference point P
along the small circle of the sphere centered on OS, where O is
the center of the sphere and S represents the azimuthal angle
parallel to the fast vibration axis of the crystal..............................
255
Figure A.3.
Figure A.4.
The relative change in polarization state induced by the analyzer
is a longitudinal shift through the arc length between the analyzer
(A) and the point of reference (P). X1 moves to X2......................... 256
Figure A.5.
The arc length from reference state P1 to final state P3 is given by 2p.
The intensity transmitted by the system is cos 2p where full intensity
is unity ........................................
257
Figure A.6.
Parallel aligned LC plate with nematic vector parallel to P.................. 258
Figure A.7.
LC cell with nematic vector 450 with respect to P and A.
Figure A.8.
The light is elliptically polarized by the sample as it moves
along the prime meridian to point E....................................
258
Effect of random inhomogenous region on the Poincare sphere
analysis. The shaded region represents a distribution of
polarization states deviating from A .......................................................
259
List of Tables
Table 1.1.
Meaning of the ratio of transmitted intensity at 00 (MIN)
to transmitted intensity at 450 (MAX)................................
.......
52
Table 2.1.
Dianhydride (R) and diamine (Z) elements of polyimides
in Figure 2.2....................................................................................... 81
Table 2.2.
Dianhydride (R) and diamine (Z) elements of polyimides
in Figure 2.4.......................................................
83
Dianhydride (R) and diamine (Z) elements of the 6F
polyimides in Figure 2.5........................................
85
Dianhydride (R) and diamine (Z) elements of polyimides
with fluorinated alkoxy side chains.................................
87
Table 2.3.
Table 2.4.
........
Table 2.5.
Measured and predicted refractive indices of SF5
polyimides .................................................... 88
Table 2.6.
Optical properties of PMDA-ODA................................
Table 2.7.
Refractive indices of optically transparent polyimides........................ 90
Table 3.1.
Color and brittleness of Probimide ® 412 films cured at
progressively higher temperatures...................................................... 108
Table 3.2.
IR absorption regions of Probimide® 412..................................
129
Table 4.1.
Color and brittleness of Probimide® 32 films cured at
progressively higher temperatures.........................................................
144
Table 4.2.
IR absorption regions of NMP and xylene........
......... 89
.............................. 161
Table 4.3.
Table 5.1
Partial list of IR absorption bands in Probimide® 32
polyamide-imide..........................
163
Composition of Merck E7 nematic liquid crystals from
Cognard's chromatographic analysis ...................................................
194
Table 5.2.
Physical properties of Merck E7 nematic liquid crystals..................... 195
Table 5.3.
Settings for EG&G lock-in amplifier used for phasesensitive detection of chopped light................................
200
List of polymers which were examined as liquid crystal
aligning agents and some of their characterisitics.....................
208
Table 5.4.
Table 5.5.
Relative ratios of the heights of the imide N-C (1370 cm- 1)
peaks to the imide C=O (1720 cm - 1) peaks for brushed
Probimide 32 layers............................................................................ 216
ACKNOWLEDGMENTS
There are so very many people I would like to acknowledge for helping this project
go forward, but knowing where to begin is not difficult at all. First, I would like to thank
my advisor Professor Peggy Cebe for seeing this project through to the end, and for her
ceaseless commitment to my graduate education. Peggy has been an immense source of
inspiration to me, as she has been to so many other students.
I would also like to thank other individuals whose work has provided direct help in
seeing this research go forward. I would like to thank Dr. Enid Sichel, not only for the
contributions she has made to the project, but also for her never-ending support and career
advice. I would also like to acknowledge UROP student Roland Desrochers for his
contributions to the project.
I am also grateful to the Minnesota Mining and Manufacturing Company for
providing funding for this project. Specifically, I would like to thank Dr. David Cross, Dr.
Robert Wenz, and Dr. Timothy Gardner for their consultations and advice.
I also thank the various individuals and institutions who donated materials and
supplies. First, I would like to thank Dr. Anne St. Clair of the NASA Langley Research
Center for providing numerous polyimide samples. I would also like to thank OCG
Microelectronic Materials, Inc., Nippon Electric Glass, Amoco Chemical Company, and
Nissan Chemical Company.
Next, I would like to express my appreciation to the following Professors and their
research groups for allowing use of laboratory equipment: Professor Anne Mayes for the
spin coater, Professor Michael Rubner for the FTIR, Professor Frederick McGarry for the
curing furnace, and Professor Stephen Senturia for the Metricon prism coupler. I would
also like to thank the individuals who helped instruct me in using the equipment.
Specifically, I would like to thank Dr. Ken Zemach for help with the FTIR, and Dr. Greg
Kellogg for help with the spin coater.
I also would like to acknowledge my thesis readers Professor Frederick McGarry,
Professor David Roylance, and Professor Lionel Kimerling. I immensely appreciate their
time and effort.
Finally, I would like to thank my colleagues, friends, and family for the love and
support they have provided. I thank all recent members of Professor Cebe's Polymer
Physics Group for providing a friendly and supportive work environment. I express my
wishes for the best of success to all of them.
Chapter 1.
Introduction and Theory
Polyimides are high temperature, high performance polymers with substantial
technological importance [1-6]. They exhibit excellent mechanical, thermal, adhesive, and
chemical-resistant properties that are highly advantageous for aerospace, electronic
packaging, display, waveguide, and other high technology applications. Many polyimides
with varying properties and behavior are available commercially.
In this thesis, I report the structure and properties of polyimides and examine their
use in liquid crystal alignment applications. It is the goal of this chapter to introduce the
reader to polyimides, and to introduce some of the main concepts that are essential for
understanding the topics addressed in this thesis. First, I discuss in this chapter the
synthesis and processing of polyimides. Then, I discuss the key material structure
issues that are pertinent to this thesis. Third, I discuss material properties, with my focus
on the optical properties of polyimides and nematic liquid crystals. Finally, I discuss the
performance of polyimides in nematic liquid crystal alignment applications.
1.1. Synthesis and Processing
1.1.1. General scheme
One synthesizes polyimides by a step growth polymerization, followed by a ring
closure reaction (see Figure 1.1) [7]. A polycondensation reaction between a dianhydride
and a diamine forms a polyamic acid, the precursor to polyimide. This reaction generally
procedes in a polar, high boiling point solvent such as y-butyrolactone (GBL, Tb = 2050 C
[8]), dimethylacetamide (DMAc, Tb = 166 0C [9]), or N-methylpyrrolidone (NMP, Tb =
2020 C [10]). A high temperature cure, usually 3000C or above, forges a ring closure, or
imidization, reaction between the acid and amide groups of the polyamic acid precursor,
converting the precursor to polyimide. One can also carry out a chemical imidization at low
temperature by adding an acetic anhydride/pyridine solution [7]. Water is removed from
the polymer chain during the imidization step.
Polyimides usually serve as film coatings. Spin coating, solvent casting, and
doctor blading are common processing techniques. Manufacturers also produce thick
extruded polyimide films. Subsequent processing of the films depends on the application.
0
11
II
C
+
0
\
H2N
\C
/H2N
11
0
Dianhydride
H
Diamine
O
H
m
0
Polyamic acid (precursor)
-H20
O
c11
-N
\C
II
O
Poy de O
Polyimide
Figure 1.1. General synthesis scheme of polyimides. PMDA-ODA shown here.
1.1.2. Photosensitive polyimide precursors
Photosensitive polyimides [11] which can be patterned by photolithography are of
tremendous interest to the electronics industry. Workers at Siemens [12,13] introduced
polyimide precursors that could be processed as negative photoresists. The photoprocessing scheme is shown in Figure 1.2. Soluble polyamic esters with photoreactive
ester groups crosslink upon exposure to UV radiation. Development of unexposed regions
yields an insoluble photopattern of crosslinked polyamic ester precursor. The precursor
material requires a high temperature cure after photoprocessing in order to convert it to
polyimide. During this step, the side chain crosslinks are released from the main chain.
Since the polymer is converted from a soluble polymer (polyamic ester) to an insoluble
polymer (polyimide), there is generally a great deal of solvent loss and film shrinkage
during this step. In addition, organic molecules which can potentially plasticize the film are
released from the polymer during the cure.
1.1.3. Preimidized polyimides
Polyimides in solution are generally in their preconverted (polyamic acid) form.
That is because the fully converted materials are usually insoluble. Workers at NASA
Langley Research Center [14] have investigated techniques of improving polyimide
solubility, which include incorporating bridging groups between phenyl rings in the main
chain and manipulating the isomeric points of attachment of the bridging groups to the
phenyl rings.
Some polyimides are highly soluble in their fully converted form and can be coated
directly from solution [15,16]. They do not need to be cured for the purpose of
imidization, but for the removal of solvent. These fully converted materials are typically
soluble in the same organic solvent as polyamic acids, i.e., NMP, GBL, etc.
One important class of polyimides are polyamide-imides [17,18], which have the
general form:
O
O
It
t
·
4N
0
c
R
RN
Z
C"
H
4
n
where R and Z are, in general, aromatic groups. Although these polymers are not widely
known to be soluble in the fully converted state [19], a fully imidized and soluble
polyamide-imide is manufactured by OCG Microelectronic Materials [18]. This material,
called Probimide 32, is of substantial importance to the research presented in this
document.
u
II1
,N•
UiR-
.
-a
C
CH3
II -uri
II
O
R = CH2 -CH20-C.C6CH2
0
o
Polyamic ester (precursor)
hv
CII" OR
- Organics
assoc. w/ R
Crosslink
OR-C4 O
Thermal Cure Step
Polyimide
Figure 1.2. Processing scheme of photosensitive polyimide precursors.
1.1.4. Preimidized, photosensitive polyimides
Researchers at Ciba Geigy in 1985 introduced polyimides that are both
photosensitive and soluble in the fully converted, imidized state [15,16]. There has been
much work done since then investigating their properties [20-27] and photo-reaction
mechanism [28-31]. Solubility in the converted state was achieved by the addition of alkyl
substituents to the polymer's phenyl groups. These alkyl groups are also involved in the
polymer's photochemical reaction. These materials can be processed as negative
photoresists from the preimidized polymer in solution without the need for imidization.
This material, unlike the photosensitive precursor, undergoes minimal shrinkage and does
not contain organic by-products upon process completion. A material of this type, its
properties, and its reaction mechanism are discussed in Chapter 3.
1.2. Structure
1.2.1. Macroscopic orientation of polymer films
Three levels of macroscopic orientation which might exist in polymer films are
depicted in Figure 1.3. Figure 1.3(a) shows a polymer which is an unoriented random
coil. This structure is completely isotropic Figure 1.3(b), on the other hand, shows a
structure that is in-plane oriented. That is, the molecular chains of the polymer
preferentially lie parallel to the plane of the film. This structure is typical of thin coated
films [32-42], especially those with rigid backbones. Polymeric films can also orient along
one axis of the film, as shown in Figure 1.3(c). One way to induce this type of orientation
in thick films is to draw the material [43-49]. In very thin films on the order of hundreds
of Angstroms, this type of orientation can be produced by brushing [50-55] the films along
one axis with a cloth. The significance of brushed polymer layers in the alignment of liquid
crystals will be discussed in section 1.4.
1.2.2. Crystalline packing
Though most polyimides are amorphous, some can be semicrystalline [56-66].
Currently, it is difficult to know for certain, prior to synthesis and experimental
determination, whether a polyimide with a certain chemical composition will readily
crystallize. In addition, since, in general, the softening and melting temperatures of these
polymers are extremely high, it is generally difficult to control crystallinity in these
materials. However, crystallinity is somewhat controllable in several thermoplastic
polyimides [59,60,62-66] designed with low glass transition and melting temperatures.
(a)
Isotropic
(b)
In-plane orientation
f
- I/
•
•
Irs-~
Uniaxial orientation
-~35~L=_
Figure 1.3. Types of orientation in polymer films: a.) isotropic orientation, b.) in-plane
orientation, c.) uniaxial orientation.
1.2.3. Chain-chain interactions
Many workers believe that charge transfer complexation [67-74], a strong
interchain electronic interaction, occurs in polyimides. Dine-Hart and Wright [67], in their
early studies of model aromatic imide compounds, first suggested this interaction to take
place. The phenomenon is demonstrated in the top portion of Figure 1.4. The lower
portion of Figure 1.4 shows the staggered arrangement of adjacent polymer chains
discussed by St. Clair [70]. Imide nitrogen donates electrons to their adjacent phenyl
groups while the carbonyl groups withdraw electrons from their adjacent phenyl rings.
Intermolecular charge transfer complexes result from the opposite partial charges on the
aromatic rings of the polymer. Furthermore, it was suggested that this strong interchain
interaction is associated with staggered packing of the phenyl rings, with rings of opposite
partial charge in maximum contact.
dianhydride
O
diamine
II
II
+
0
Chain 1
+
Figure 1.4. Charge transfer interactions in polyimides.
Chain 2
1.2.4. Liquid crystalline nematic order and alignment
Nematic order
Monomeric nematic liquid crystals [75-78] are very important materials for display
applications. A typical chemical structure [77,78] of such a material is shown below:
B
In the above structure, "A" represents a chemical group such as an ester, a phenyl ring, or,
perhaps, no group. "B" is typically a hydrocarbon chain 3-7 carbon atoms in length.
The nematic liquid crystalline state is a condition in which the molecules in a phase
orient along a preferred axis know as the nematic director. The nematic state is depicted in
Figure 1.5, where D' is the nematic director. Further discussion of the nematic phase can
be found in many texts [75,76].
pD'
Figure 1.5. A nematic liquid crystalline phase with director D'.
Nematic alignment
The difference between nematic order and nematic alignment is a crucial distinction.
A slab of nematic liquid crystalline material will form many nematic grains, each with its
own nematic director. But without any external influence, the nematic directors of the
individual grains will have no preferential orientation with respect to the bulk. This is
represented in Figure 1.6 (a).
Macroscopic "alignment" [77-81] is the condition in which the nematic directors of
the liquid crystals in a slab of material are non-random with respect to the bulk. That is, the
liquid crystals are preferentially oriented along a particular axis of the bulk. A "perfectly"
or "ideally" aligned specimen will be entirely homogeneous, that is, have only one nematic
director. This condition is represented in Figure 1.6(b). The ability to align liquid crystals
on the macroscopic level is essential for using of liquid crystals as flat panel display
materials.
Alignment can be produced, in a general sense, by a number of techniques [77-81].
Such techniques include applying an electromagnetic field, shearing, inducing flow, and
treating adjacent surface layers. In this thesis, the treatment of adjacent surface layers to
produce nematic liquid crystal alignment is of particular interest, and will be discussed later
in this chapter (section 1.4) as well as in Chapters 5 and 6.
1.3. Properties
1.3.1. Quantitative relationships between polymer chemistry and refractive index
The index of refraction determines the speed of light in a material through the
defining equation:
n(c) = v/c
(1)
where n is the refractive index, co is frequency, v is the speed of light in the material, and c
is the speed of light in a vacuum.
Refractive index is also a measure of the electronic polarizability of the material.
The index of refraction is related to electronic polarizability (a) by the Lorentz-Lorenz
equation:
[(n2 - 1) / (n2 +2)] (M/p) = (Nav/3co) a
(2)
where M is molecular weight, p is density, Nay is Avagadro's number, eo is relative
permittivity in a vacuum. Refractive index is related to dielectric constant by:
(a)
\1
ss
_o
J•
s
S
14
(b)
D' - an axis of the bulk
Figure 1.6. Representations of nematic liquid crystalline material.
b.) bulk alignment along axis D'.
a.) no alignment, and
Eo = n2
(3)
where e, is the dielectric constant at optical frequency.
The refractive index is an important parameter for polyimide applications. For
many electronic packaging applications, a low dielectric constant is required for minimizing
crosstalk between the copper lines and maximizing signal propagation through the copper
itself. The index of refraction is a particularly valuable measure of dielectric properties
because it is easily obtained experimentally, even if the material is anisotropic. Polyimide
electronic packaging layers have also been suggested to serve as waveguides for optical
interconnect technologies [82]. The refractive index is an important parameter in evaluating
their performance as waveguides.
Researchers have found that fluorinated polymers have low refractive indices. The
index of refraction, for example, for polyethylene (-CH 2 CH 2) is 1.51 [83], for poly
(vinylidene fluoride) (-CF2CH 2-) is 1.42 [83], and for polytetrafluoroethylene (-CF 2CF 2-)
is 1.35 [83]. Groh and Zimmermann [84] predict that the intrinsic lower limit for the
refractive index that can be introduced by fluorination is about 1.29. A tremendous amount
of work has been done in reducing the refractive indices of polyimides by addition of
fluorine to the polymer backbone [85-99].
Because the index of refraction is an important physical property of a material, the
relationship between polymer chemical structure and index of refraction is of significant
interest to polymer scientist. In addition, since polyimides with low refractive indices are
of significant industrial interest, it is important to understand why the index of refraction is
reduced. This is an important subject of Chapter 2 of this thesis.
Techniques have been developed to predict the index of refraction of a polymer,
given its chemical structure. Two methods are discussed below. The first class of
techniques discussed here are van Krevelen additive group contribution techniques [100].
The second type of approach is a prediction methodology developed by J. Bicerano [101]
which utilizes the concept of atomic bond connectivity [102,103].
Van Krevelen
The underlying concept behind van Krevelen [100] group contribution techniques is
that polymer properties are "in some way determined by a sum of contributions made by
the structural and functional groups in the molecule or repeating unit of the polymer"[100].
The individual contributions, called "increments", that each of these groups makes to a
physical property are assumed to be additive.
33
The additive physical property we are concerned with here is the "molar refraction",
from which refractive index can be easily calculated. Van Krevelen offers three techniques
for refractive index prediction [100]. The first is based on the Lorentz-Lorenz equation,
previously shown in equation (2), and shown below in equation (4).
[(n2 - 1) / (n2 +2)] (M/p) = (Nav/3eo) aO P
RLL
(4)
P is the molar polarization defined here as equal to the molar refraction RLL. The molar
refractions of the individual van Krevelen groups (i) in the backbone are summed according
to:
RLL = X RLLi
(5)
It is crucial to note that foreknowledge of the molar volume M/p is required in order to
make this calculation.
The second technique is based on the equation of Gladstone and Dale [104]:
RGD = (n- 1) (M/p)
(6)
The molar refraction RGD is summed in the same way, using a different tabulated set of
group contribution values for the groups that make up the backbone. The molar volume
M/p is also required to make this calculation.
The third technique does not require experimental values for the density nor any
other experimental measurement. This method is based on the equation of Vogel [105]:
Rv = nM
(7)
which states that the Vogel molar refraction RV is simply the product of refractive index (n)
and molecular weight (M) in grams/mole. The total Rv is the summation of the individual
group contributions.
A limitation of this technique is that it does not explicitly take into account the
density of the material. Nonetheless, this equation has been shown to give refractive index
predictions which are as accurate as those obtained by the more theoretically-based
equations of Lorentz-Lorenz and Gladstone-Dale. The Vogel technique is hence a
convenient way to estimate refractive index without the need for experimental density data,
which is often unavailable. This technique, however, does not factor in the possibility of
34
increased density due to semicrystallinity, chain packing, or other type of ordering. We
might therefore expect this method to be more accurate for amorphous polymers with
limited chain packing.
One limitation of this form of technique, in general, is that contributions of a
particular group may differ in different surroundings. For example, the bonding
environment around a particular group may differ if attached to an alkyl group as opposed
to an aromatic group. However, the technique attempts to correct, as much as possible, for
different bonding environments.
Another limitation of the van Krevelen method, in general, is that the functional
groups are preselected and may not be appropriate or convenient for complicated novel
polymers. A reasonable approach to assembling an appropriate set of contributing groups
must be employed, and sometimes best guesses must be made. For example, correction
factors for several ring structures have been calculated by van Krevelen, although no value
is specifically available for the imide ring structure present in polyimides. Since most ring
structures have very close correction factors, the value for a similar ring, a piperidyl ring, is
used instead in Chapter 2. A sample van Krevelen/Vogel calculation for polystyrene is
shown in Figure 1.7.
Bicerano
A new method for polymer property prediction developed by Jozef Bicerano of
Dow Chemical Company has recently become available [101]. This is not a group
contribution method, but a topological method that takes into account all of the atoms and
bonds present in the material. It utilizes four connectivity indices [102,103] and other
parameters which are calculated as described below and demonstrated for polystyrene in
Figure 1.8. Because there is no preselection of chemical contribution groups, we should
expect this technique to be especially useful for complex polymers such as polyimides.
The hydrogen suppressed repeat unit is drawn (with a bond drawn at only one end
of the repeat unit connecting the unit to the next one). At the location of each atom, the
number of non-hydrogen atoms to which it is connected is written. The value at each atom
represents the zeroeth order simple connectivity index 8 of the atom. The zeroeth order
simple (atomic) connectivity index for the entire molecule OX is then the sum of the negative
one-half powers of the connectivities of all its atoms:
ox = 1 (1/I)
( 8)
Van Krevelen Calculation (Polystyrene)
20.64
CH-
(ar)
21.4
--0
123.51
Rv = 165.55
M = 104 g/mol
n = R, /M = 1.592
nexp = 1.591
Figure 1.7. Sample calculation of the index of refraction of polystyrene using the van
Krevelen / Vogel technique. The predicted value, 1.591, is in excellent agreement with the
experimental value 1.592 [83].
The product of the atomic connectivities of every two adjacent atoms is then written
at the bond line between the two atoms. The value at each bond line represents its bond
index p. The first order simple (bond) connectivity index for the entire molecule is then
calculated as shown in equation (9). This index, though used to predict other properties, is
not present in Bicerano's refractive index correlation.
'X = I (1/•p)
(9)
Next, on a new hydrogen suppressed repeat unit, zeroeth order valence connectivity
indices 8•, given by equation (10), for each atom, are written at the location of each atom.
8v = (Zv - NH) / (Z - Zv -1)
(10)
In equation (10), Zv is the number of valence electrons, NH is the number of adjacent
hydrogen atoms, and Z is the atomic number. For second row atoms carbon, nitrogen,
oxygen, and fluorine, equation (10) reduces to 8v = Z v - NH, that is, the atom's valence
minus the number of hydrogen atoms attached. First order valence connectivity indices for
the bonds of the molecule are calculated as the products of the adjacent zeroeth order
valence indices and are written at the bond lines. Atomic and bond valence connectivity
indices for the entire atom are calculated as shown in equations (11) and (12).
OXV = e (1/~8v)
(11)
1xv = I (1/4V)
(12)
In the development of correlations for property prediction [101], Bicerano uses
these connectivity parameters, as well as additional correction factors which account for the
overestimation or underestimation of the contribution of certain groups or atoms due to, for
example, orbital conjugation, hydrogen bonding, or unusually low polarizability.
Bicerano's correlation for predicting refractive index at room temperature is shown in
equation (13).
n = 1.8853 + 0.024558 (17 OXV - 20 OX -121v. - 9 Nrot + Nref) /N
(13)
In equation (13), N is the total number of non-hydrogen atoms. Nrot is the number of
rotatable, non-aromatic single bonds in the repeat unit that change the spacial coordinates of
one or more atoms upon rotation. Bonds in "floppy" rings contribute one-half to Nrot.
Nref is a correction factor given by equation (14).
Nref = -11NF - 3NCI-Ph + 18NS + 9 Nfused + 12NHB + 32NSi-si
(14)
In equation (14), NF is the number of fluorine atoms, NS is the number of sulfur atoms,
NCI-Ph is the number of chlorine atoms bonded to phenyl rings, Nfused is the number of
rings which are present in attached-edge multi-ring (e.g., aromatic imide) structures, NHB
is the number of hydrogen bonding moieties, and NSi-si is the number of delocalized and
conjugated silicon-silicon bonds. Only Nfused and NF are relevant to the polyimides
discussed in this work.
Using 183 polymers with refractive indices ranging from 1.34 to 1.69, a correlation
coefficient of 0.977 and a standard deviation of 0.016 was obtained by Bicerano for
equation (10). The standard deviation represents less than 1% of the average refractive
index of the data set. Thus, we should expect this method to be highly accurate.
1.3.2. Property changes associated with charge transfer complexation
Charge transfer complexation (see section 1.2.3) is believed to have a substantial
effect on the properties of polyimides [67-74,85-99]. Examples of property changes
believed to be associated with charge transfer complexation include increased yellow
coloration, glass transition temperature, dielectric constant, refractive index, and optical
attenuation. Workers have found that charge transfer complexation is mitigated when
separator groups, chain kinks, and bulky chemical substituents are introduced into the
polyimide backbone [72-74,85-99]. Introduction of these chemical groups have marked
effects on properties. For example, St. Clair et al. [73,74,89,96] have found that
incorporation of charge transfer-reducing groups reduce coloration and dielectric constant.
Fryd [72] found that addition of such groups reduce glass transition temperature. Feger et
al. [98] have found that addition of bulky fluorinated groups reduces optical losses in
waveguides.
Bicerano Calculation (Polystyrene)
6
Repeat unit
Simple Connectivity
Valence Connectivity
8v = valence minus
# adjacent H
8 = # adjacent
non-H atoms
pijv = 8V x 8j
•j = 68 x 8j
N=8
6
o = 1 (1/48) [atomic connectivity index
Nrot =-3
Nref -
I X= (1/ 3) bond
= (6/N2) + (2/N3)
=
connectivity index
o
5.397
= (1/42) + (6/q3) + (1/N4)
=
1 = (4/N4) + (4/46) + (1/49)
= 3.966
4.671
X Y = (2/q6) + (4/49) + (3/N12)
= 3.016
Nref = -11NF - 3 NCI-Ph + 18Ns + 9 Nfused + 12 NHB + 32 Nsi.si
n = 1.885312 + 0.024558 x (17 OXv - 20 OX -12 IXV - 9 Nrot + Nref) / N
n (predicted) = 1.604
n (experimental) = 1.592
Figure 1.8. Bicerano prediction for the refractive index of polystyrene. The estimated
value, 1.604, is within 0.75% of the experimental value, 1.592 [83].
1.3.3. Anisotropy of optical properties
Birefringence is a term which is defined as a difference in refractive index in a
material along different spacial axes. However, it is very important that the term not be
used loosely. That is, we must be precise in how we define the axes of comparison. In
Figure 1.9(a-e), we review a number of ways in which birefringence can be defined.
Examples of optical anisotropy on three different levels are presented: molecular,
microscopic, and macroscopic.
Molecular birefringence
Optical anisotropy on the molecular level is shown in Figure 1.9(a). In the
molecule in this figure, the refractive index along the chain axis n// is different from the
refractive index perpendicular to the chain axis n±. One might define the molecular
birefringence Anmol as:
Anmol = n// - n±
(15)
In this particular example, n// should be greater than nj. This is because the polarizability
of the aromatic ring is greater in its plane than normal to it. The axis parallel to the main
chain must always be in the plane of the phenyl rings of the polymer, while axes
perpendicular to the main chain will, in general, not be. For a molecule in which the highly
polarizable aromatic units are perpendicular to the main chain, e.g., polystyrene, n// will be
less than n± [33,34].
Microscopic birefringence
Birefringence on the microscopic level is shown in Figure 1.9(b,c). A polymer
spherulite appears in Figure 1.9(b) in which the refractive index along the radial axis nr is
different from the refractive index along the tangential axis nt. It might be convenient for
the case in Figure 9(b) to define birefringence of the spherulite Ansph as:
Ansph = nr - nt
(16)
The relationship of nr to nt will depend upon the orientation of the main chain with respect
to the radial axis, as well as the relationship between n// and nl described above.
Figure 1.9(c) shows a nematic liquid crystalline grain in which the refractive index
along the extraordinary axis ne is different from the refractive index along the ordinary axis
no. This is another form of microscopic birefringence. We might define birefringence of
the liquid crystalline phase AnLC as:
AnLC = ne - no
(17)
We can expect ne to be, most often, greater than no because the molecules of organic liquid
crystals typically have their primary polarizable groups along the long axis of the molecule.
Macroscopic birefringence
Birefringence on the macroscopic level is shown in Figure 1.9(d,e). An in-plane
oriented polymer film is shown in Figure 1.9(d) in which the bulk in-plane index nTE is
different from the bulk out-of-plane index nTM. This film structure was previously shown
in Figure 1.3(b). The difference between nTE and nTM will depend upon whether the main
chain is more polarizable along its main chain or normal to its main chain. If, for example,
the molecule is more polarizable along its main chain (e.g., polyimide), nTE will be greater
than nTM, as the bulk film consequently will be more polarizable in the plane of the film.
On the other hand, if the polymer is more polarizable normal to the main chain (e.g.,
polystyrene), nTM will be greater than nTE [33,34]. Here, we can define the birefringence
of the film Anfilm as:
Anfilm = nTE - nTM
(18)
An aligned liquid crystal film is shown in Figure 1.9(e) with differing refractive
indices nTE,//, nTE,I, and nTM. The condition of macroscopic alignment was previously
shown in Figure 1.6(b). In fhe case that the liquid crystals are perfectly aligned, nTE,//
should be equal to no in Figure 1.9(c), while nTE,± and nTM should be equal to ne from
Figure 1.9(c).
polymer molecule
-0CD
(b)
nt
n
polymer spherulite
Figure 1.9. Examples of optical anisotropy. a.) birefringence on the molecular level, b.)
birefringence on the microscopic level in a polymer spherulite,
(c)
nematic liquid crystalline domain
no
(d)
n TM
in-plane oriented polymer film
nTE
riTE
TM
aligned nematic liquid crystaline film
nTE,
S
TE,//
Figure 1.9. continued. Examples of optical anisotropy. c.) birefringence on the molecular
level in a nematic grain, d.) bulk birefringence in an in-plane oriented film, e.) bulk
birefringence in a film of aligned liquid crystals.
For the general case of a macroscopic birefringence, we can imagine a block of
material in Cartesian axes as shown in Figure 1.10. We can define three refractive indices
nx, ny, and nz, as well as three macroscopic birefringences as shown in equations (19),
(20), and (21):
Anxy = nx - ny
(19)
Anxz = nx - nz
(20)
Anyz = ny - nz = Anxz - Anxy
(21)
We can also define an average refractive index as:
navg = (1/3) (nx + ny + nz)
(22)
For an in-plane oriented polymer film which is isotropic in the plane of the film, as shown
in Figures 1.3(b) and 1.9(d), average refractive index is:
navg = (2 nTE + nTM) / 3
(23)
This focus of this thesis is macroscopic optical properties. However, others [106] have
done extensive investigations of the relationships between intrinsic molecular birefringence,
crystalline fraction, crystalline orientation function, and macroscopic birefringence.
Molecular vs. microscopic vs. macroscopic birefringence
It is important to realize that a material can be birefringent on either the molecular,
microscopic, or macroscopic level, yet not be birefringent on any of the other two. For
example, most polymers have refractive indices that are different along the main chain as
compared to normal to the main chain. However, a film or block of such a polymer will
not necessarily be birefringent on the macroscopic or even the microscopic level. It may be
entirely amorphous and unoriented.
Likewise, a material can be birefringent on the microscopic level, but not on the
macroscopic level. Figure 1.10 shows a block of polymer which is semicrystalline. The
crystallites are perfectly spherical. Assuming that, for the spherulites, nr • nt, the material
will be highly birefringent on the microscopic level. When placed between crossed
polarizers, a significant quantity of light will be transmitted. However, if one measures the
bulk refractive indices along axes x, y, and z of the block, one will find that they are
exactly the same! Between crossed polars, rotation of the block about the normal axis will
not change the transmitted intensity. It is does not matter whether, for example, the x-axis
is 00 or 450 to the polarizer. That is, we have the situation in which the material is highly
birefringent (microscopically), though the bulk refractive indices are identical in every
direction.
The situation described above may occur for any situation in which there is
microscopic birefringence, but no orientation with respect to the bulk. A slab of unaligned
nematic liquid crystals, for example, will be "birefringent" locally, but "non-birefringent" if
we define the optical axes with respect to the bulk sample. A slab of aligned liquid
crystals, on the other hand, will be birefringent on both the microscopic and macroscopic
levels.
perfectly spherical
crystallites
0 m
polymer
block
/I
0000 03
Uh
I I,.
Figure 1.10. A thick semicrystalline polymer block that is microscopically birefringent but
has nx=ny=nz.
1.3.4. Measurement of index of refraction
There are many techniques in existence that can be used to measure the refractive
index of a material [107-110]. These include ellipsometry, Abbe refractometry, and fluid
matching. Another technique of recent interest is called waveguide mode prism coupling
[108,109], and is discussed here.
The application of the prism coupling technique is shown in Figure 1.11. The film
is pressed to the base of a high index prism. A polarized laser is focused on one face of the
prism. The light refracts into the prism, totally internally reflects at the base of the prism,
and is detected as it leaves the opposite face. When the angle of incidence e is equal to that
of an allowed mode in the film, the light propagates into the film and there is a sharp drop
in light intensity at the detector. Also, when O drops below the critical angle of the polymer
film, the laser refracts into the film, and there is a large permanent drop in detected
intensity. The refractive index of the material can be calculated from either the waveguide
mode "coupling" angles or from the critical angle. The critical angle approach, while
slightly less accurate than waveguide propagation method, is preferable to use in thick films
in which waveguide modes are poorly pronounced.
Workers at Bell Labs developed the prism coupling method in the early 1970s
[108]. Today, a commercial instrument, the Metricon prism coupler, applies this
technique. Using the Metricon instrument, one can measure both the in-plane index (nTE)
and out-of-plane index (nTM) of a thin film by changing the polarization of the incident
light. The Metricon prism coupler is equipped with both waveguide mode and critical angle
measurement capability. Many workers have used the prism technique to measure the
refractive index and optical anisotropy of polyimide and other polymer films [32-41,43].
FILM
Figure 1.11. Prism coupling technique for measuring index of refraction.
1.3.5. Optical transmission model for liquid crystal alignment
This section is dedicated to the discussion of an adequate framework for evaluating
the alignment of nematic liquid crystals in a film. Nematic liquid crystals are birefringent
on a microscopic level. The molecules in each liquid crystalline domain are oriented
preferentially along a director with the index of refraction greater along the ordinary axis
than along the extraordinary axis. The microscopic birefringence of the liquid crystalline
phase is a function of the material itself and is independent of any external forces which
may act to align the liquid crystals with respect to the bulk. However, in this section, we
are concerned with evaluating the macroscopic alignment of the liquid crystals, that is, how
well they are oriented with respect to a given axis of the bulk.
Optical model for an aligned liquid crystal plate
Let D' be the nematic director of an aligned liquid crystal plate (such as that found
in Figure 1.9(e)), with D" the ordinary axis orthogonal to D' and in the plane of the plate.
Also, let OP be the axis of the polarizer, and OA be the axis of the analyzer. Let angle X be
the angle between the polarizer and analyzer, angle 0 be the angle between the polarizer and
nematic vector. These vector are shown in Figure 1.12 below.
Figure 1.12. Graphical representation of a parallel-aligned liquid crystal plate between a
polarizer (P) and analyzer (A).
An electric field plane wave passing through the polarizer enters the liquid crystal
plate. When D' is not parallel or perpendicular to OP, the electric field vector is divided
into two components that have mutually orthogonal displacement vectors but differing
velocities of propagation. The two waves emerge from the plate with a phase difference 5
given by [111]:
(24)
8 = (2n / X)(An) h
where:
,
= wavelength of light in free space
An
= difference between ordinary and
extraordinary refractive indices
h
= plate thickness
The intensity I of light passing through the crystal plate, polarizer, and analyzer is
given by [111]:
I = E2 {cos 2 x - sin 20 sin 2(4-X) sin2 (8/2))
(25)
When the polarizer and analyzer are crossed, i.e. X= nt/2, intensity II is given by:
II
= E2 sin2 24 sin 2 (8/2)
(26)
When the nematic vector D' is aligned parallel or perpendicular to the polarizer axis
OP, the value of Ij is identically zero. That is, according to equation (26):
II = Ij, min = 0,
when
4 = 0, 7/2,
in, ...
(27)
On the other hand, the angular position of the liquid crystal nematic vector which
produces maximum intensity II, max is 450 with respect to the polarizer axis. The
maximum intensity in equation (26) is given by:
II = Ij, max = E2 sin2(8/2), when
4 = n/4,
37r/4, 57t/4, ...
(28)
If we normalize the equation for I1() given by equation (26) by the maximum
intensity II, max given by equation (28), we get the following simple expression for
I(O)/I(f/4), that is, the intensity at angle 4 normalized by the maximum intensity found at
S----n/4:
I(0) / I (c/4) = sin 2 (20)
(29)
This relationship is plotted below in Figure 1.13.
0
in
l,~I
10
>1
C
.O1
.01
.001
.0001
0
45
90
135
Angle (degrees)
180
Figure 1.13. Plot of I(0)/I(450 ) vs. 0 from equation (29).
Elliptically polarized light
Figure 1.14 shows the most general case of elliptically polarized light
[107,112,113]. The direction of polarization systematically changes in an elliptical pattern
as it propagates. The polarization state of the electric field can be described by three
parameters, which are represented in Figure 1.14. The first parameter is the handedness,
right (clockwise) or left (counterclockwise). The second parameter is the azimuth a, that
is, the angle between the semimajor axis of the ellipse and the defined reference axis. The
third parameter is the ellipticity 0, which is the ratio between the lengths of the semimajor
and semiminor axes of the ellipse, or b/a in Figure 1.14.
b
left handed
a
right handed
Figure 1.14. Elliptically polarized light with handedness (left or right), azimuth (a), and
ellipticity (P = a / b).
Linearly and circularly polarized light are special cases of elliptically polarized light.
Linearly polarized light is the case where 3 = 0. It can possess any value of a, but has no
handedness. Circularly polarized light, on the other hand, is the case where 101 = 1. It can
be left or right handed, but has no azimuth.
Misalignment
In the case of a perfectly aligned specimen, polarized light passing through the
specimen will move from an original state of linear polarization (a = 0 and 1 = 0) to
another state of elliptical polarization depending upon the thickness of the film and the
orientation with respect to the polarizer. This is true except for the case where the liquid
crystals are aligned 0 or 900 with respect to the polarizer and in which there is no change in
the alignment state. Now consider a film of liquid crystals in which the liquid crystals are
not in a perfect state of alignment. A case in which there are regions of liquid crystals
misoriented with respect to the bulk alignment axis is depicted below in Figure 1.15.
Direction of alignment
Figure 1.15. Parallel-aligned liquid crystals with inhomogeneities.
Consider only the case in which the polarizer and analyzer are crossed and the
liquid crystal alignment director is parallel to the polarizer. We showed previously that
under these conditions in an ideally aligned system, no light is transmitted, i.e., intensity is
zero. However, if the system has misaligned liquid crystal domains, some light can be
transmitted as a consequence of random elliptical polarization produced as light travels
through the unaligned regions. Thus, unlike the ideal case, transmitted intensity will be
greater than zero.
Each distinct misaligned nematic domain will have its own orientation with respect
to the polarizer. The following exercize will clarify this concept. Imagine a linearly
polarized laser beam in azimuth ao passing through a LC specimen with random
inhomogeneities "a", "b", "c", "d", and "e". This is shown in Figure 1.16. As the laser
penetrates the liquid crystalline sample at point "1", the light passes through inhomogenous
phases "a" and "b". Both "a" and "b" have unique nematic directors and unique optical
path lengths. Light incident at point "2" of the specimen, however, travels through
inhomogenous phases "c", "d", and "e", which have their own unique directors and optical
path lengths. The light emerging at points 1' and 2' will have unique ellipticities 3 1 and 12
and unique azimuths al and a2 different from the initial polarization.
A large beam of light, hence, will emerge from the sample with a distribution of
polarization states depending upon the significance of the misalignment. Consequently,
some light will not be blocked by the analyzer. Furthermore, since there is no specific
relationship between P1and 12 or between al and a2, there is no compensator, e.g., a 1/4
wave plate, that can single-handedly negate the ellipticity of the emanating light.
P=E0 a:=czo
LASER:
--a
1
•
I
V
zy
I
12
w|
m
21
02, (X2
01, al
I1
•2
•0
al # a2 # 0X
Figure 1.16. Rays of light passing through different points 1 or 2 of the liquid crystal
sample and emerging at 1' or 2' with different polarization states. Domains a-e represent
regions of differing optical retardation.
Characterizing the Misaligned State
Let us suppose that a liquid crystal film is placed between a crossed polarizer and
analyzer with the alignment axis at an angle 0 with respect to the polarizer. Let us now
define a liquid crystal alignment parameter "A", which is given by the ratio of the
=00 to the experimentally measured
experimentally measured transmitted intensity at --
transmitted intensity at 0=450. The parameter "A" is, essentially, the ratio of minimum to
maximum intensity.
A - I(00) / I(450)
(30)
The meanings of the values of A are shown in Table 1.1. It has already been
shown in equation (29) that the ideal value of A for a perfectly aligned liquid crystal film is
zero. From the previous discussion of misalignment, it is clear that for a film with
misalignment, A will be greater than zero. For the case that there is no alignment, A is
unity, as there is no distinguishable difference between the 00 and 450 conditions. Hence,
A will, in general, be a parameter between zero and one inclusively, with low values
representing strong alignment, high values representing weak alignment, zero representing
perfect alignment, and one representing no alignment. Note that a value of A greater than
one would represent the case in which the material is aligned with a greater preference
along the 450 axis than along the alignment director that was initially assumed.
Table 1.1. Meaning of the ratio of transmitted intensity at 00 (MIN) to transmitted intensity
at 450 (MAX).
MIN/MAX (A) ratio
Meaning
A=0
LCs perfectly aligned
0 <A < 1
LCs have intermediate level of alignment
e.g., A is small, but > 0
LCs strongly aligned
A is large, but < 1
LC weakly aligned
A= 1
LCs not aligned
A> 1I
LCs aligned with preference to 450 axis
Using equations (29) and (30), we can also define an "effective angle of
misalignment" O:
A = sin 2 (20)
(31)
The parameter 0 represents the angle at which the liquid crystals appear to be misaligned
with respect to the alignment (00) axis. For the case that A=0, i.e., perfect alignment,
0--=0. For the case that A=1, i.e., no alignment, 0=450. The 0-=450 condition, means that
the liquid crystal nematic phases have their directors oriented along any axis with equal
probability. Equation (31) is not valid for the odd case that A>1.
1.4. Performance: Liquid Crystal Alignment Layers
Polyimides are important as electronic packaging materials, as engineering plastics
for high technology applications, and as alignment films for liquid crystals. Much of this
thesis focuses on their use as liquid crystal alignment films. A discussion of the alignment
of liquid crystals on polymer surfaces follows.
1.4.1. Liquid crystal surface alignment on brushed polymer films
The surface alignment of liquid crystals is a crucial element in the performance of
LCD technology [77-81]. "Alignment layers" at surfaces of liquid crystal films, as shown
in Figure 1.17, define the axis of macroscopic liquid crystal orientation at that surface.
Alignment layers are typically rubbed polyimides or obliquely evaporated SiOx thin films.
In the case of rubbed polyimides, it has been observed that liquid crystals that come in
contact with a rubbed polymer film align in the direction the polymer is rubbed.
NEMATIC LIQUID
CRYSTALS
S
- -. -
---
-F
B
BRUSHED
POLYMER
ALIGNMENT
FILM
AXIS OF BRUSHING
Figure 1.17. Alignment of liquid crystals on brushed polymer surfaces.
The liquid crystals can order in the direction of the rubbing at an inclination angle
with respect to the horizontal surface. This is known as the angle of surface pretilt
[114,115]. Depending upon the particular device technology, a specific pretilt angle can be
desired for optimal switching performance. The liquid crystal pretilt angle is a function of
the polymer alignment material, the nematic liquid crystal, and the way in which the
alignment films are processed [116-119]. The generation and control of pretilt was not,
however, investigated in this thesis.
The brushing process is accomplished industrially on a spinning wheel covered
with a silk, rayon, or other cloth. This is shown in Figure 1.18. On the laboratory scale,
however, films can be brushed by hand to create liquid crystal alignment, and complex
apparatus is not required.
BUFFING
WHEEL
I
POLYMER
SURFACE
Figure 1.18. A polymer surface being rubbed by a brushing wheel.
Although rubbed polyimides perform as excellent liquid crystal alignment layers,
the mechanism of alignment is not well understood. Brushing-induced surface alignment is
often referred to as a "black art", as it has both baffled and intrigued scientists for decades.
Berreman and others [120-122] have postulated that liquid crystals align along surface
grooves created by the rubbing process. More recently, workers have shown that liquid
crystal alignment is more likely associated with the orientation of chemical groups in the
alignment polymer [48-55]. Geary et al. [50] observed that polymers that are oriented and
highly crystalline are effective liquid crystal alignment films. As to why liquid crystals
would align on oriented surface is a matter of speculation, though it has been suggested
[123] that intermolecular interactions between the polymer and the liquid crystal are
involved.
1.4.2. Alternative treatments to polymer layers for liquid crystal alignment
There are a number of serious disadvantages of the brushing process as a means of
inducing alignment of liquid crystals. First, the process can generate particulates which can
affect the quality of display devices. Second, the buffing can create static charge, which
can interfere with the device electronics. Third, the brushing can only be performed on
planar surfaces. Notched or curved substrates can present difficulties in achieving
brushing-induced alignment. Fourth, brushing is a costly and energy consuming
mechanical process. Finally, brushing is a surface-uniform technique which cannot be
used directly to pattern a device.
A number of alternative, non-contact liquid crystal surface alignment processes
have been proposed and demonstrated. For example, workers have investigated the noncontact formation of surface grooves as a means of producing alignment [124,125]. Also,
Langmuir-Blodgett films have also been used to align liquid crystals [126-128].
Recently, Schadt and others [129-131] have demonstrated the alignment of liquid
crystals on films of a photosensitive cinnamate polymer (polyvinylmethoxycinnamate)
[132] that have been exposed to linearly polarized ultraviolet light (LP-UV). Schadt [129]
proposes that an anisotropic structure, as shown in Figure 1.19, is forged in the polymer
alignment film during the exposure to LP-UV. The top portion of Figure 1.19 shows the
chemical groups which react, while the bottom portion depicts the proposed oriented
structure. The polymer, which crosslinks upon exposure to UV, is believed to crosslink in
a preferential way upon exposure to LP-UV which results in an anisotropic distribution of
crosslinked side chain molecules, exerts an orienting force upon the main chain of the
polymer, and depletes uncrosslinked side chains oriented along the LP-UV axis. The
authors believe that the induced orientation in the polymer is associated with the liquid
crystal alignment, which was found to be perpendicular to the axis of the LP-UV.
Other researchers have investigated linearly polarized light as a way producing
alignment in liquid crystals. One might refer to this class of techniques as "photoalignment". Ichimura et al. [133-135] have investigated the alignment of liquid crystals on
azobenzene films that have been exposed to linearly polarized UV. The azobenzene dyes
undergo a 'trans-cis' photoisomerization upon exposure to the linearly polarized light
[136,137]. Gibbons et al. [138-142] have investigated the optical control of liquid crystal
alignment using azo dye doped polyimide alignment films and exposing liquid crystal cells
to linear polarized laser beams. Gibbons' group is interested in photo-orientation processes
not only for display applications, but also for holography, optical data storage, and other
potential applications of liquid crystals which require their macroscopic alignment.
o0
LP-UV axis
.o
LP-UV axis
Figure 1.19. The photochemical reaction in polyvinylmethoxycinnamate and the oriented
structure, proposed by Schadt [129], that is formed by exposure to linearly polarized UV.
Hasegawa and Taira [143] demonstrated the alignment of nematic liquid crystals on
films of an aliphatic/aromatic polyimide that have been exposed to linearly polarized deep
ultraviolet light. They found alignment to occur, however, in only a tiny window of
exposure time, below and above which, no alignment was seen. The authors suggest that
the anisotropic depolymerization of the polyimide film is responsible for the alignment.
They argue that at short exposures, the polymer begins to degrade preferentially along the
LP-UV axis, as shown in Figure 1.20. This process results in an oriented structure in the
polymer film that is preferential to the axis in the plane of the film perpendicular to the LPUV. At the exposure dosage critical for producing alignment, the polymer film is in a state
of optimal orientation. At this point, chains parallel to the LP-UV become depleted and
further exposure to LP-UV destroys chains perpendicular to the UV axis as well, thus,
destroying the alignment.
LP-UV AXIS
!
1400,
K
w
ORIENTATION
AXIS
Figure 1.20. Anisotropic chain degradation proposed to occur [143] in a polyimide film
during exposure to linearly polarized UV.
There is precedent for the belief that deep UV depolymerizes the polyimide. For
example, Kambe [144] discusses the degradation of mechanical properties of aromatic
polyimide, polyamide, and polyamide-imide films that have undergone very long
exposures to deep UV. In addition, polyimides are well known to be destroyed during
deep UV laser ablation processes [145-149]. In analogy, the LP-UV here might be thought
of as a light, anisotropic surface ablation.
1.4.3. An LCD device: the TN-LCD
Liquid crystals displays are devices which take advantage of the optical properties
of macroscopically oriented liquid crystalline materials in order to produce switchable
pixels. So that it is clear why surface alignment is important in the display industry, an
example of a simple liquid crystal device, known as the TN-LCD, or "twisted nematic
liquid crystal display" [150], is briefly explained here, and also shown in Figure 1.21. It
should be noted, however, that there are other types of LCDs [151], as well as other
potential applications of liquid crystals.
A TN-LCD cell consists of a nematic liquid crystal phase which is gradually rotated
by 900. The 900 rotation is achieved by inducing surface-alignment directors at each liquid
crystal-polymer interface that are perpendicular to each other. The twisted liquid crystal
film is sandwiched between two substrates, each coated with the appropriately treated
alignment film. Each substrate contains a polarizer with the axis of polarization parallel to
alignment director defined by the adjacent alignment film. There are also electrodes atop
each substrate so that an electric field can be applied to the cell.
In the "off" state, the twisted TN layer rotates the direction of polarization of the
entering light by 900. The second polarizer prevents light from passing through, and the
cell appears dark. However, when voltage is applied to the two electrodes on each surface
of the TN layer, the liquid crystals reorient parallel to the applied field. In that state, the
layer no longer twists the direction of polarization of the light passing through, and the cell
appears bright. When the electric field is removed, the liquid crystals return to their default
alignment state, which is the twisted nematic state. Also note that if the two polarizers in
the device were perpendicular instead of parallel, the cell would function in reverse, i.e., as
off/on instead of on/off.
Light cannot pass
ayer
4
Light passes through
Polarizer
Substrate
Electrodes
,coated with
alignment layer
Substrate
Polarizer
state:
(b) On
voltage applied
Light
Figure 1.21. States of a twisted nematic liquid crystal display (TN-LCD) device. a.) on
state and b.) off state.
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2.
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C. Feger, M.M. Khojasteh, and J.E. McGrath, Eds., Polyimides: Materials,
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Chapter 2.
Refractive Indices of Polyimides
2.1. Introduction
The optical index of refraction is an important physical property of polyimides. The
purpose of this chapter is to explore the fundamental factors that determine the indices of
refraction of polyimides as a class of materials. The work focuses primarily on the
relationship between polyimide chemistry and refractive index, but also investigates various
processing effects.
Refractive index is a measure of the material's electronic polarizability, which is an
important component of its dielectric behavior. In many electronic packaging applications,
a low dielectric constant is required for minimizing crosstalk and maximizing signal
propagation speed. Refractive index is related to the dielectric constant by:
Eo = n2
(1)
where n is refractive index and eo is the dielectric constant at optical frequency. The index
of refraction at optical frequencies, unlike the dielectric constant at electrical frequencies, is
easily obtained experimentally, even if the material is anisotropic.
The index of refraction is also important as a waveguide parameter. Polyimide
electronic packaging layers have been suggested to serve as waveguides for optical
interconnect technologies [1].
It is well known that polyimides with high fluorine content [2-19] have low
dielectric constants and refractive indices. Unfortunately, however, there is no adequate
explanation in the literature as to why this is so. There are three possible explanations:
reduced crystallinity, reduced electronic polarizability, and reduced charge transfer
complexation.
First, it is possible that bulky fluorinated groups impede crystallization in
polyimides. Index of refraction is related to density as shown in the Lorentz-Lorenz
equation shown below:
[(n2 - 1) / (n2 +2)] = (p/M) (Nav/3eo) x
(2)
where M is molecular weight, p is density, Nay is Avagadro's number, so is relative
permittivty in a vacuum. In equation (2), for the range of refractive indices typical of
organic polymers, refractive index increases with (p/M) and with a. Crystalline polymers
are more densely packed and would therefore, in general, be expected to have higher
indices of refraction. Though crystallinity has been observed in some polyimides [20-29],
polyimides are most frequently considered to be primarily amorphous. Thus, reduced
crystallinity is probably not a fully adequate explanation for the reduced indices of
refraction in fluorinated polyimides.
Another possible explanation is reduced electronic polarizability. Refractive index
is related to electronic polarizability a as previously stated in equation (2). The carbonfluorine bond has low electronic polarizabilty, and consequently, fluorinated polymers are
known to have low indices of refraction [30].
A third explanation is the reduced charge transfer complexation associated with the
incorporation of bulky fluorinated groups. The groups sterically hinder the intermolecular
interactions in the polyimide. This theory was put forth by St. Clair et al. [2,10-12].
It is quite possible that all three of these explanations play some role in the reduced
indices of refraction. However, it has been entirely unclear as to what are the relative
contributions of these effects. Discussions of this issue in the literature are frustratingly
inadequate.
For example, Inchino et al. [3] systematically increased the length of fluorinated
alkoxy side chains in order to systematically reduce the dielectric constant of polyimides.
The authors state, in agreement with St. Clair et al. [2,10-12], "the increase in length of the
fluorinated alkoxy groups, leading to a more bulky separator which sterically hinders the
[chain-chain] interaction, must reduce the dielectric constant " [3]. The authors then
continue to state that "fluorine has low electronic polarizability which reduces the dielectric
constant" [3]. However, it is unclear in this work what they believe is the main
contributor.
The explanation put forth by Matsuura et al. [15,16] for the reduced dielectric
constant in rigid rod fluorinated polyimides is more confusing. According to the authors,
"the low dielectric constants of the fluorinated polyimides result from reduced chain-chain
interaction due to the low electronic polarizability of the fluorine" [16]. This is, of course,
a non-sensical explanation, as the low electronic polarizability of the C-F bonds and the
reduced intermolecular interactions are different and unnecessarily related phenomena.
Hence, it is the goal of this chapter to gain better insight into the mechanisms of the
refractive index reduction in polyimides that occurs as a result of fluorination. The optical
properties of a large number of polyimides are compared. Experimental refractive indices
are then compared to indices calculated by the property prediction techniques described in
Chapter 1. The effects of processing on the index of refraction in polyimides are also
investigated later in the chapter.
2.2. Experimental Techniques
2.2.1. Polyimide film preparation
Two sources of polyimide refractive index data were used. The first was the
literature [2-5,31-36]. Films preparation for these samples are described by the authors.
The second source was polyimide films synthesized at and provided by Dr. Anne St. Clair
of the NASA Langley Research Center, Hampton, Virginia. The chemical compositions
of these and other polymers are presented in Tables 2.1-2.3. Polyimides ODPA-3,3'ODA, ODPA-4-BDAF, 6FDA-3,3'-ODA, 6FDA-4-BDAF, 6FDA-DASP, BFDA-DASP,
BPDA-DASP, ODPA-DASP, IPAN-DASP, BTDA-DASP, HQDEA-DASP, BDSDADASP, BTDA-1,3BABB, PMDA-ODA, and ODPA-PDA were obtained as thick free
standing films. The polyamic acids were synthesized in N,N-dimethylacetamide, solution
cast on glass, and cured in a low humidity environment at 100 0 C, 2000 C, and 3000 C for
one hour at each temperature. PMDA-ODA was annealed an additional 20 minutes at
4000 C. RegulusTM NEW-TPI, whose chemical composition is shown in Table 2.1, was
received from the Mitsui Toatsu Chemical Co. Polyimides discussed in section 2.4.3,
PMDA-ODA, PMDA-APB, PMDA-BDAF, 6FDA-ODA, 6FDA-APB, and 6FDA-BDAF,
were received as 15% solids by weight solutions in N,N-dimethylacetamide. The
solutions were spin coated on glass using a Headway Research spin coater at 4000 RPM
for 30 seconds. The coated films were progressively cured at 100 0 C, 200 0 C, and 3000 C
for one hour at each temperature.
2.2.2. Zone drawing
The zone drawing technique has been described previously [37-40] and is depicted
in Figure 2.1 below. The film is placed between two clamps with a weight pulling on one
end. A zone heater passes over the film at a constant rate which heats the film above the
glass transition temperature. The material is drawn at the site of heating. By varying the
drawing weight and temperature, films of various draw ratios (final length / initial length)
were produced for 6FDA-4-BDAF fluorinated polyimide films.
Zone Heater
>Tg
I
Grinp
V
Grips
Polymer film
Dead Weight Load
Figure 2.1. Film zone drawing method.
2.2.3. Refractive index measurement
Refractive index measurements were made for polyimide films using a Metricon
PC-2000 prism coupler with X = 6328A. Average refractive indices were calculated as:
navg = (1/3) (2nTE + nTM)
(3)
The prism coupling technique for measuring index of refraction is discussed in Chapter 1.
2.2.4. Refractive index prediction
Two property prediction techniques, which are discussed in Chapter 1, were
utilized in order to estimate how varying chemical make-up should affect refractive index.
It is assumed that these estimation techniques account for the intrachain electronic
polarizability of the polymer molecular repeat unit. The first is the group contribution
method by van Krevelen [41] based on an equation used by Vogel [42]. The second is a
topological technique by Bicerano [43]. Both methods, unlike two other techniques
described by van Krevelen [41], require no measurement of the polymer density prior to
the calculation. Only the chemical structure is needed. However, since polymer density is
not used in these calculations, both methods are intended only for amorphous polymers.
For details concerning these calculations, please refer back to Chapter 1 or the original
references [41-43].
2.3. Relationship Between Polyimide Chemistry and Refractive Index
Experimental and predicted refractive indices of a large number of polyimides were
compared. Three types of behavior were found. First, many amorphous, non-fluorinated
polyimides were found to have refractive indices of around 1.68-1.70, deviating slightly,
though systematically, above the predicted refractive index. Second, some polyimides
were found to have refractive indices that were substantially higher than those predicted.
Third, fluorinated polyimides had low refractive indices which generally compared
favorably to predicted values.
2.3.1. Amorphous, non-fluorinated polyimides
A comparison between predicted and measured refractive indices in nonfluorinated
polyimides is shown in Figure 2.2. The polyimides have generally been arranged from
left to right in order of declining measured refractive index. The chemical structures of the
polyimides are shown in Table 2.1.
Although the calculated refractive indices are close to experimental values, we
notice a systematic underprediction, especially when using the presumably more accurate
method of Bicerano. Refractive indices calculated by van Krevelen's method are good,
underpredicting by only about 0.02-0.03. Bicerano's method underpredicts refractive
index by about 0.05-0.06, which is three to four times the standard deviation in
Bicerano's correlation. It is unlikely that such large deviations from experimental values
in the Bicerano computations can be attributed solely to statistical error.
Charge transfer complexation [2,10-12,44-49] and/or molecular ordering might
result in deviations of predicted from measured refractive indices. There has been no
evidence of substantial crystal lattice ordering in these materials. However, as shown in
Figure 2.3(a), PMDA-ODA may exhibit slight paracrystalline order when examined using
wide-angle X-ray scattering (WAXS). Alternatively, chain packing which might result
from intermolecular complexation could be responsible for an increase in density of the
amorphous phase and/or an increase in electronic polarizability. The property prediction
techniques apparently do not account for the chain packing and/or charge transfer effects.
2.3.2. Semicrystalline polyimides
The index of refraction of a polymer is directly related to its density. Since
crystalline phases are denser than amorphous phases, a measured refractive index that is
substantially higher than that predicted by property prediction methods may be associated
with polymer semicrystallinity. Figure 2.4 compares predicted and measured indices of
polyimides with unusually high refractive indices. See Table 2.2 for the chemical
structures.
There is substantial experimental evidence of crystal lattice ordering in these
particular materials. Previous authors have shown that BPDA-PDA [22], ODPA-PDA
[20], BTDA-1,3-BABB [21,23-27] are crystallizable. WAXS analysis, shown in Figure
2.3(b), confirms the presence of a crystalline phase in our ODPA-PDA sample. This
material has exactly the same atoms and bonds as does PMDA-ODA (chemical structure
Table 2.1, WAXS Figure 2.3(a)), but has a refractive index significantly greater. Thus,
there is likely a strong structure-property relationship between crystallinity and refractive
index in polyimides which is not taken into account in the van Krevelen and Bicerano
predictions.
2.3.3. Polyimides containing hexafluoroisopropylidene (6F)
The optical properties of polyimides containing the hexafluoroisopropylidene (6F)
group have been the subject of much investigation [2-19]. It has generally been observed
that the refractive indices of polyimides decrease with increasing fluorine content. It is
also believed by these authors that the 6F group, which consists of two trifluoromethyl
groups bonded to carbon, sterically inhibits charge transfer complexation in the
polyimides.
Figure 2.5 shows a comparison of predicted and measured refractive indices of
polyimides containing the 6F group. For the chemical structures, see Table 2.3. Both
property prediction methods estimate refractive index quite accurately. Bicerano's method
is accurate when the 6F group appears in both the dianhydride and diamine portions of the
repeat unit, but very slightly underpredicts refractive index when the 6F group exists in
only one segment. This demonstrates that the lower refractive indices that have been
observed experimentally in 6F materials can be predicted from the modified chemical
structure. Furthermore, the good agreement between measured refractive index and these
predictions suggests that chain packing and charge transfer complexation may be reduced
in these materials.
2.3.4. Polyimides containing fluorinated alkoxy side chains
The synthesis and characterization of novel polyimides containing fluorinated
alkoxy side chains, shown in Table 2.4, have been reported [3]. Measured refractive
indices and dielectric constants were shown to be reduced systematically with increasing
fluorine content. The fluorine content of the polyimides was increased by increasing the
length of the side chains.
In Figure 2.6, predicted refractive index (Bicerano technique) is shown as a
function of fluorine content. The predicted indices compare fairly favorably to
measurement. However, experimentally, a slight dependence on the dianhydride chemistry
was found by the authors. Polyimides containing the 6FDA dianhydride, for example, had
lower refractive indices than polyimides containing non-fluorinated dianhydrides, but equal
overall fluorine content. This is not predicted, however, by Bicerano's method, as shown
in Figure 2.6. This may indicate that the 6F group is more effective per fluorine atom than
is the side chain in reducing polymer chain packing.
2.3.5. Polyimides containing pentafluorosulfanyl (SF 5)
Recently, St. Clair and St. Clair [19] reported the synthesis of novel fluorinated
polyimides containing a pentafluorosulfanyl (SF5) group. Measured and predicted
refractive indices of these polymers are shown in Table 2.5. We found a small deviation
between predicted and experimental values. The least deviation occurred in a polyimide
containing a bulky isopropylidene group in the dianhydride portion of the chain (IPAN).
Likely, the SF5 group impedes chain packing to some extent, though not as well as the 6F
group.
1.8I 9
£
Measured
van Krevelen
Ricerann
•IAVrl
"M·v
1.7:
A
a
0
0
O
0
D£
r
O
O
0
n
v
0v
1.6
1.5 I
II
I I
I
--
I
---
III
I
I
5
Polyimide
Figure 2.2. Experimental and predicted refractive indices of non-fluorinated polyimides in
Table 2.1.
C
C
(I
10
20
30
40
TWO THETA (DEGREES)
Figure 2.3. Wide angle X-ray scattering (WAXS) of: a.) PMDA-ODA and b.) ODPAPDA polyimide films. The polymers contain the same exact atoms and bonds, but have
very different physical structures, and correspondingly, very different optical properties.
Table 2.1. Dianhydride (R) and diamine (Z) elements of polyimides in Figure 2.2.
0
C
-N
O
\
R
6
N-Z-
6
1.8-1
1.7
S0o
O
o
1.6 - A Measured
1 van Krevelen
o Bicerano
1.5
Polyimide
Figure 2.4. Measured and predicted refractive indices of semicrystalline polyimides in
Table 2.2.
83
Table 2.2. Dianhydride (R) and diamine (Z) elements of polyimides in Figure 2.4.
84
A Measured
O Van Krevelen
o Bicerano
1.7
0 0A0
1.6
1
0
0
o~oos
,
0
0
ooo
go
A
at
0
0
i
I
I
a
A
1.5
I
I
I
I
I
I
I
Polyimide
Figure 2.5. Measured and predicted refractive indices of 6F polyimides from Table 2.3.
E
0
71
(NJ
0
(N-
Uo
z>
0C
H:E
o
*1
86
--
1.8t
x
"O
1.7-
-
0
I..
,-
O 6FDA
C)C
A BPDA
1.6-
O BTDA
0o
1.5
"4
10.
1.4
SI
0
10
I
20
30
I
I
40
50
60
Fluorine content (wt. %)
Figure 2.6. Predicted refractive indices (Bicerano's method) of polyimides with
fluorinated alkoxy side chains vs. fluorine content. Chemical structures in Table 2.4.
Table 2.4. Dianhydride (R) and diamine (Z) segments of polyimides with fluorinated
alkoxy side chains, reported by Inchino et al. [3].
R
Z
--
CF3
CH2 (CF2)3 F
NJ
6FDA
OCH 2 (CF2)7 F
BPDA
O
II
CH2 (CF2)6 F
4-'--C
BTDA
CH2 (CF2)10 H
Table 2.5. Measured and predicted refractive indices of SF5 polyirnides.
SFs
O
-N
R
C
N.
C0
Polyimide
C
OI
0
R
CF63
6FDA-DASP
CCF3
CF3
CF3
BFDA
nmeas.
1.541
nBic.
1.536
nvK
1.529
1.576
1.559
1.554
1.640/
1.592
1.582
DASP
,
BPDABPDADASP
1.635*
ODPADASP
1.620
1.588
1.576
1.597
1.583
1.570
1.621
1.588
1.577
1.624
1.594
1.586
,CH
P 3
CHI3,
IPAN-DASP
O
II
BTDADASP
C
HQDEA-
BDSDA-
0
Z O.
Z.O
. 1.647 1.616 1.602
* First value represents side 1, the second value, side 2.
2.4. Relationship Between Polyimide Processing and Refractive Index
2.4.1. In-plane birefringence in polyimide films
PMDA-ODA
PMDA-ODA is a polyimide that is widely used commercially. The repeat unit of
the polymer is shown below:
O
O
-N
N
C
II
C
II
O
O
Figure 1.3 shows three types of orientation in polymer films: isotropic orientation, in-plane
orientation, and uniaxial orientation. Thin films of PMDA-ODA have been found to have"
out-of-plane refractive indices, nTM, that are significantly lower than in-plane refractive
indices, nTE [31,32]. This is because the main chain (c) axes lie preferentially in the plane
of the film as shown in Figure 1.3(b). The average refractive index, navg = (2nTE + nTM) /
3, and birefringence, An = nTE- nTM, of fully annealed spun cast PMDA-ODA films were
reported by Russell et al.[31]. The authors found navg = 1.695 and An = 0.08. Data for
our PMDA-ODA samples are shown in Table 2.6. Solvent cast film received from the
NASA Langley Research Center had a birefringence of 0.078, while commercial DuPont
T
Kapton m
H had a birefringence of 0.111. The commercial film, as compared to the NASA
Langley film, had a lower out-of-plane refractive index nrM and a higher in-plane refractive
index nTE, which suggests that the film has a greater level of in-plane ordering. The
average refractive indices, however, of both films were around 1.7.
Table 2.6. Optical Properties of PMDA-ODA.
Material source:
nav
NASA Langley
1.697
DuPont Kapton M H 1.702
inTE
nTM
An
1.723
1.645
0.078
1.739
1.628
0.111
Optically transparent polvimides
Refractive indices of four optically transparent polyimides synthesized at the NASA
Langley Research Center are listed in Table 2.7. Chemical structures can be found in
Tables 2.1 and 2.3. Unlike the PMDA-ODA films, very little optical anisotropy is seen in
these materials. We conclude that there is little preferential in-plane alignment of the
polymer chains in these films, as shown in Figure 1.3(a). This may be a result of either the
high thickness of the films or a lack of a tendency of these materials to order in the plane of
the film.
Table 2.7. Refractive indices of optically transparent polyimides.
Name
ODPA-3,3'-ODA
ODPA-4-BDAF
6FDA-3,3'-ODA
6FDA-4-BDAF
nTM
1.683
1.615
1.598
1.566
nTE
1.681
1.612
1.597
1.564
Thickness
23tm
28pm
34pm
34gm
2.4.2. Birefringence in zone drawn films
Nakagawa [32] and Smirnova and Bessonov [34] report the optical properties of
uniaxially cold drawn PMDA-ODA. They show that the refractive index is highest in the
direction of drawing and lowest normal to the direction of orientation. This is because the
polarizable groups in the main chain are preferentially oriented along the draw axis, as
shown in Figure 1.3(c). Both authors report a maximum birefringence on the order of
0.20. The average refractive index, however, is unaffected by the level of orientation, as it
is near 1.7, which is consistent with what was previously mentioned in Section 2.4.1.
The chemical structure of 6FDA+4-BDAF, a fluorinated, low refractive index
polyimide, is shown below:
CF,3
,CF3
When the material is zone drawn along one axis of the film, significant optical anisotropy is
observed. Figure 2.7 shows the refractive index of 6FDA+4-BDAF in three perpendicular
directions as a function of draw ratio.
In the direction of the draw in the plane of the film, refractive index increases with
draw ratio. Refractive index decreases with draw ratio, however, out of the plane of the
film or in the plane of the film perpendicular to the direction of the draw. This is, again, a
result of the preferential alignment of the primary polarizable groups along the main chain
in the direction of the draw. The average refractive index is not affected by zone drawing.
Refractive index is lowest out of the plane of the film. At high draw ratios, it is nearly as
low as 1.54, which is about 0.017 less than the average index of the film.
1.60
It
- A€"
S1.58
a
A in-plane/parallel to draw
A
aI.
1.56-
E in-plane/normal to draw
O
0
0
D
o
oO
0 out-of-plane
1.54
.1-
1
2
Draw ratio
3
Figure 2.7. Refractive index vs. draw ratio for 6FDA+4-BDAF in the direction of the
draw and normal to the draw.
2.4.3. Effect of cure temperature on refractive index
As explained in Chapter 1,polyimides generally begin as polyamic acid precursors,
which are thermally converted to polyimides. As the polyimides undergo this reaction, the
refractive index of the material increases. This is demonstrated in Figure 2.8, which shows
the average indices of refraction of several polyimides as a function of cure temperature.
The films were spin coated from solutions obtained from NASA Langley, and their
chemical structures can be found in Tables 2.1-3. Note that, as discussed previously,
refractive index decreases with increased fluorine content.
To investigate the reasons for the increase in refractive index with increased cure
temperature, the predicted refractive indices of the polyamic acid precursors were calculated
using the Bicerano technique. In Figure 2.9(a), these values are compared to the measured
refractive indices of the 100 0 C cured materials. Apparently, the values are in reasonably
good agreement, regardless of whether the polyimide contains the 6F group. According to
Kan and Kao [50], the imidization reaction does not occur appreciably until about 150 0 C,
but occurs rapidly above 250 0C. It is thus reasonable to assume that the 100 0 C treatment is
representative of the unconverted state, while the 3000C is representative of the imidized
state.
In Figure 2.9(b), predicted refractive indices of the fully converted polyimide are
compared to measured refractive indices after the 300 0 C cure. As found in the previous
sections, the Bicerano underpredicts the refractive indices of the non-fluorinated
polyimides, i.e., PMDA-ODA and PMDA-APB, but is in good agreement with the
refractive indices of the 6F fluorinated polyimides, i.e., the remaining four polymers.
The good agreement with predicted values for the polyamic acids suggests that there
are no unexpected chain packing or charge transfer effects in the precursor materials that go
unaccounted for by the Bicerano prediction technique. However, when the materials are
converted to polyimide, the non-fluorinated materials undergo a significant unpredicted
increase in refractive index. Since this increase in refractive index cannot be accounted for
based upon the chemical change alone, this study suggests that the imidization reaction
leads to greater chain packing and/or increased interchain interaction in the non-fluorinated
polyimides. The 6F materials, on the other hand, remain consistent with predicted values
upon imidization, indicating that the 6F group impedes the chain packing.
1.75
I
~
~I~
IC_
_ 1_1_
1.70-
PMDA-ODA
--e---
1.65-
1.60~--=s~-~
~==;--~ft===z~~
1 _ ~--~-~t---------6
-I
-
1.55
1.50
rl
1.50-
I ---100
200
Temperature
-----
PMDA-BDAF
--
6FDA-ODA
6---
---- o--
6FDA-APB
--
6FDA-BDAF
*--
--
- ---
PMDA-APB
-
-
300
(0C)
Figure 2.8. Refractive indices of several polyimides as a function of cure temperature.
-1
a
A
100 0 C (exp.)
o Bicerano (calc.)
1.7
1.60o
b0
.
..
. i.
.
..
.
.
Polyamic Acid
Figure 2.9. Predicted (Bicerano's method) vs. measured refractive indices of several
polyimides. a.) Predicted values of the polyamic acid precursors vs. measured values of
the films after only 100 0Ccure.
3
b
3000 C (exp.)
Bicerano (calc.)
A
o
1.7
o
1.6
o
A
0
0
2
Polyimide
Figure 2.9. continued. b.) Predicted values of the polyimides vs. measured values after the
300 0 C cure.
2.4.4. Effect of post-cure annealing on NEW-TPI polyimide
Regulus TM NEW TPI [28,29] is a polyimide that is known to crystallize when
annealed above its glass transition temperature (Tg = 2500 C). Crystallinity, as previously
discussed, can increase the index of refraction of a polymer. Several samples of fully
cured NEW-TPI were annealed in order to crystallize the material. Films were treated for
10 minutes at various temperatures between 2800 C and 3200 C, and for various times at
2900 C. The annealing process roughened the surface of the films, thus making it more
difficult to take accurate data using the Metricon prism coupler. Each refractive index (nTE
or nTM) is therefore the average of four measurements.
Both the average refractive index and birefringence (nTE - nTM) increased with
annealing temperature, as shown in Figure 2.10. There is a significant step in average
refractive index and birefringence at 2900 C, indicating that this could be the onset
temperature of rapid crystallization. The increase in bulk birefringence suggests that the
crystals may have preferentially order in the plane of the film.
The refractive index and birefringence also increase with annealing time at 2900 C,
as shown in Figure 2.11. Average refractive index appears to begin increasing after about
three minutes of annealing, appears to be about half way to its maximum near six minutes,
and appears to level after about 10 minutes. Previously reported work [28] for NEW-TPI
samples isothermally annealed at 300 0 C, indicates a crystallization half time (ti/2) of about
6.3 minutes and a crystallization completion time (tc) of about 12 minutes. The general
increase in birefringence with annealing time also suggests that the crystals may have inplane order.
2.5. Conclusions
Significant variations of the refractive index of polyimides have been observed with
varying, often subtly varying, chemical structure. Property prediction calculations of
refractive indices of polyimides give some insight into the causes of these changes. While
charge transfer complexation effects may slightly increase the refractive indices of
polyimides, semicrystallinity or short range ordered structure can increase the refractive
index substantially. Incorporation of fluorinated groups is an effective means of reducing
the refractive index. The observed reduction results from a combination of reduced
electronic polarizability and reduced interchain interaction and/or chain packing. Refractive
(a)
-1.
*o
W
1
•=
1.
.z3
I
1W
tW
I.
-·I
As rec
300
290
280
310
320
Temperature (OC)
(b)
0.025
0.020 -
0.015 0.010 0.005·.·. :i:
:::::::::;:::::
0.000
:.::
::::
I6-"""w
-
As rec
280
290
=
Mý
"""""***
300
*
"0 ...WOMMIN.
bw
""""""
310
320
Temperature (OC)
Figure 2.10. a.) Average refractive index of NEW-TPI vs. post-cure annealing
temperature. b.) Birefringence vs. post-cure annealing temperature.
1.72
1.71
1.70
1.69
1.68
1 .(
I
I
I
I
I
0
1
2
3
4
I
5
6
Minutes at
I
I
I
I
7
8 9
0
290 C
10 11 12
8
0
290 C
10
(b)
0.025
0.020
0.015
0.010
0.005
0.000
0
2
4
6
Minutes at
12
Figure 2.11. a.) Average refractive index vs. post-cure annealing time at 290 0C. b.)
Birefringence vs. post-cure annealing time
index prediction techniques appear to be highly accurate for polyimides with bulky
fluorinated substituents, but underpredict for non-fluorinated polyimides.
In addition to chemical structure, polyimide processing can affect the index of
refraction. Oriented materials, for example, will be birefringent. While uniaxially drawing
the films will, of course, induce orientation, birefringence can sometimes be induced
simply by the casting or annealing process. The overall refractive index can also be
dependent upon the annealing conditions, as both amic acid to imide conversion and
polymer crystallization increase the refractive index.
2.6. References
1. R. Selvaraj, H.T. Lin, and J.F. McDonald, J. Lightwave Tech., 6(6),1034 (1988).
2.
A.K. St. Clair and T.L. St. Clair, in Polymersfor High Technology - Electronicsand
Photonics, p. 437, M.J. Bowden and S. R. Turner, Eds., ACS, Washington (1987)
3. T. Inchino, S. Sasaki, T. Matsuura, and S. Nishi, J. Polm. Sci: Polym Chem., 28,
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4.
R. Reuter, H. Franke, and C. Feger, Applied Optics, 27(21), 4565 (1988).
5.
G. Hougham; G. Tesoro; A. Viehbeck; J. Chapple-Sokol, Polym. Prep, 34(1), 371
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6.
S. Saski, T. Matsuura, S. Nishi, and S. Ando, in Materials Science of High
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7.
J.W. Labadie, M.I. Sanchez, Y.Y. Cheng, and J.L. Hedrick, ibid., p. 43.
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100
9.
D.M. Stoakley and A.K. St. Clair, in Optical and ElectricalPropertiesof Polymers,
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(1989).
18. D.L. Goff, E.L. Yuan, H. Long, and H.J. Neuhaus, ibid., p. 93.
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20. M.K. Gerber, J.R. Pratt, and T.L. St. Clair, in MaterialsScience of High
TemperaturePolymersfor Microelectronics,p. 487, D.T. Grubb, I Mita, and D.Y.
Yoon, Eds., Mater. Res. Soc. Proc., 227 (1991).
101
21. P.M. Hergenrother and S.J. Havens, in MaterialsScience of High Temperature
Polymersfor Microelectronics,p. 453, D.T. Grubb, I Mita, and D.Y. Yoon, Eds.,
Mater. Res. Soc. Proc., 227 (1991).
22. M. Ree, T.L. Nunes, and D.P. Kirby, Polym. Prep., 33(1), 309 (1992).
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Chem., 25, 1093 (1987).
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26. J. Friler and P. Cebe, Polym. Eng. Sci., 33(10), 588 (1993).
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28. P.P. Huo and P. Cebe, Polymer, 34(4), 696 (1993).
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30. W. Groh and A. Zimmermann, Macromol., 24, 6660 (1991).
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(1983).
32. K. Nakagawa, J. Appl. Polym. Sci., 41, 2049 (1990).
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102
34. V.E. Smirnova and M.I. Bessonov, in Polyimides: Materials, Chemistry, and
Characterization,edited by C. Feger, M.M. Khojasteh, and J.E. McGrath, Elsevier,
Amsterdam, 1989, p. 563.
35. S. Herminghaus, D. Boese, D.Y. Yoon, and B.A. Smith, Appl. Phys. Lett., 59(9),
1043 (1991).
36. D. Boese, H. Lee, D.Y. Yoon, J.D. Swallen, and J.F. Rabolt, J. Polym. Sci.:
Polym. Phys., 30, 1321 (1992).
37. T. Kinugi, C. Inchinose, and A. Suzuki, J. Appl. Polym. Sci., 31, 429 (1986).
38. Y. Aihara and P. Cebe, Polym. Prepr., 33(1), 471 (1992).
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40. J.B. Teverovsky, D.C. Rich, Y. Aihara, and P. Cebe, J. Appl. Polym. Sci., 54, 497
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Vysokomol. Soyed., A19(3), 614 (1977)
46. J.M. Salley, T. Miwa, and C.W. Frank, in Materials Science of High Temperature
Polymers for Microelectronics,p. 117, D.T. Grubb, I Mita, and D.Y. Yoon, Eds.,
Mater. Res. Soc. Proc., 227 (1991).
103
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Hergenrother, Eds., Blackie, Glasgow (1990).
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104
Chapter 3. Curing Study of Probimide® 412 Preimidized
Photosensitive Polyimide
3.1. Introduction
Over the past decade and a half, there has been substantial interest in photosensitive
polyimides (PSPIs) [1]. The photosensitive behavior allows the polyimides, which are
widely used in electronics applications, to be patterned photolithographically. The first
type of PSPI developed was a photosensitive polyamic ester [2], which crosslinks only in
the precursor state. The second type was an intrinsically photosensitive polyimide [3,4]
which is both solvent soluble and photosensitive in its fully converted state. It is the latter
version of the PSPI with which we are concerned.
In general, polyimides require high temperature cures for conversion of the
precursor polymer and for the removal of solvent. The chemical and physical properties of
polyimide systems change substantially as they are cured, and the material is converted
from precursor to polyimide. Because the intrinsically photosensitive polyimides are
already in their fully converted state, and because they gel upon exposure to ultraviolet
light, they show strong potential as high performance materials which can be cured rapidly
and at low temperatures. One application in which rapid, low temperature curing is highly
desirable is in their use as alignment films for liquid crystal display devices [5,6].
There has been much work published regarding the performance and physical
properties of intrinsically PSPI films that have undergone ultraviolet exposure and
conventional high temperature thermal cures [7-14]. Ree et al. [14] found that chemical
crosslinks are formed by both photochemical and thermal curing. In the present work, we
show how thermal and ultraviolet curing, which do not involve closure of the imide ring,
physically and chemically change the material. We do so by examining a series of films
that have undergone step cures from soft bake to hard bake. We report the effects of the
atmosphere in which the hard bakes are performed, and the effects of ultraviolet exposure
on the films.
105
3.2. Experimental
3.2.1. Polymer film samples
The polyimide system studied is of the Probimide® 400 series produced by OCG
Microelectronic Materials, Inc. The polymer contains the following photosensitive
structural unit:
0
_l
Isb
u
P
-N
I
0II
where R, R' = alkyl group. The polyimide contains a benzophenone (Ph-CO-Ph) group in
the dianhydride portion of the repeat unit. The polyimide is obtained in y-butyrolactone
(GBL) solvent, and the product used in this study, Probimide® 412, has a 12.5% original
solids content by weight.
Bulk films of Probimide® 412 on the order of 100 gpm thick were produced. All
films were amorphous, and exhibited no Bragg scattering peaks when examined by wide
angle X-ray scattering.
The film casting procedure is depicted in Figure 3.1. The polyimide solution was
first dropped onto a glass plate and wiped with a metal doctor blade with a 1 mm gap as
shown in Figure 3.1(a). The wet films were dried to a tack free state under a dry air
current at room temperature overnight as shown in Figure 3.1(b). The parent films were
then systematically treated at higher temperatures. Samples were cut from the film after
each curing step. The first treatment was 4.5 hours at 100 0C in air. The second was 3
hours at 200 0 C in air. Films were then treated at 300 0 C for three hours in either air or
nitrogen. Material treated at 3000C in nitrogen was then additionally cured at 4000 C for one
hour in nitrogen. Any Probimide® 412 material treated at 400 0C in air was essentially
destroyed by oxidation. A separate film was treated at 140 0 C for 3 hours in air after the 4.5
hour bake at 100 0C. These thermally cured samples were not exposed to ultraviolet
irradiation.
Material that had undergone the 4.5 hour treatment at 100 0C was irradiated under a
Spectroline model BIB-150B ultraviolet lamp with X = 364 nm and an area normalized
power of approximately 2.5 mW/cm 2. The film was exposed to approximately 27 J/cm 2
106
over the course of 3 hours, each side of the film being exposed for 90 minutes. In order to
separate the effects of hard bake and UV cures, we did not subsequently hard bake the
irradiated film.
Thin films of Probimide® 412 were spun cast from the original solution in GBL on
glass or KBr at 4000 RPM for 30 seconds. The samples were immediately softbaked at
100 0C for fifteen minutes in air. The samples appeared dry after softcuring. The thin films
were then progressively cured at 2000 C, 300 0C, and 400 0C, one hour at each temperature.
The 200 0C cures were performed in air, while the 3000 C cures were performed in either air
or nitrogen. The 400 0 C cures were performed in nitrogen only on films previously treated
at 3000 C in nitrogen. Spin cast films treated at 100 0C were irradiated under the UV lamp
for various times.
3.2.2. Measurement techniques
A Seiko TG/DTA320 was used for thermogravimetric analysis (TGA) of the bulk
films. Samples were scanned from room temperature to 575 0 C at 100C/minute in flowing
air (300 ml/minute).
Dynamic mechanical analysis (DMA) was performed in tension mode on bulk films
using a Seiko DMS200. Samples were scanned under nitrogen at 20C/minute from room
temperature to the temperature above softening at which breakage occurred. Frequencies 1,
2, 5, 10, and 20 Hz were used. Only the 1 Hz data is presented here, as the frequency
dependence of the DMA spectra was unnoteworthy.
Index of refraction at X = 632.8 nm was measured using a Metricon PC-2000 prism
coupler. The prism coupler technique is discussed in Chapter 1. Refractive index was
measured by determination of the waveguide propagation modes. Both in-plane (nTE) and
out-of-plane (nTM) indices were measured. The average refractive index navg was
calculated as shown below:
navg = (1/3) (2nTE + nTM)
(1)
Refractive index measurements were taken for the free standing films (by critical angle) and
the films spin coated on glass (by waveguide mode determination).
Fourier transform infrared spectroscopy (FTIR) was performed using a Nicolet
510P. FTIR spectra were taken for the Probimide® 412 films spin coated on KBr.
107
SGlass
plate
blad
Dotor
with 1mm gap
(b)
Glass plate with coated
polymer film
Figure 3.1. Prepartion of bulk films. a.) Coating and blading process for optimization of
film smoothness and thickness uniformity. b.) Pre-cure air-drying chamber.
108
3.3. Results And Discussion
3.3.1. Coloration And Brittleness
The changes in coloration and brittleness in Probimide® 412 thick films that occur
with subsequent cures are summarized in Table 3.1. Probimide® 412 is slightly yellow
and flexible when cured at 1000 C. These characteristics change little upon curing to 2000 C.
The material becomes dark orange and brittle when cured at 300 0 C in air, but remains
yellow and flexible in nitrogen. However, the polymer turns brown and brittle when
treated at 400 0C in nitrogen. The dark coloration and brittleness in the films cured at 300 0C
in air and at 400 0C in nitrogen suggest that chemical crosslinking is taking place.
Ultraviolet irradiation after thermally curing at 100 0C substantially increases the yellowness
of the film. The yellowing of the UV irradiated film is indicative of the formation of
chemical crosslinks.
Table 3.1. Color and Brittleness in Films Cured at Progressively Higher Temperatures.
Treatment
100 0C
2000C
3000C
300 0C
400 0C
100 0C+UV
Environment
air
air
N2
air
N2
air
Color
light yellow
light yellow
light yellow
dark orange
brown
yellow
Brittleness
flexible
flexible
flexible
brittle
brittle
barely brittle
3.3.2. Thermogravimetric Analysis
Thermogravimetric analysis was used to track the presence of solvent as the bulk
films are cured. Thermogravimetric scans in Figures 3.2 and 3.3 also indicate vaporization
of absorbed water and, at high temperatures, decomposition of the polymer. All of the
films lose about 1% of their original weight below 150 0C due to removal of water from the
polymer, while the films lose varying amounts of weight, depending upon the cure
conditions, between about 150 0 C and 300 0C due to removal of residual solvent. The TGA
scans reach a plateau until the temperature exceeds about 400 0C, when the films begin to
degrade.
109
In Figure 3.2(a-e), five thermogravimetric scans of Probimide® 412 films are
compared. Weight loss (left vertical axis) and derivative of weight loss (right vertical axis)
are shown as functions of temperature. Each sample has a different highest curing
temperature which is indicated by the vertical marker. None of the films in Figure 3.2 has
been UV cured. The amount of residual solvent vaporized between 150 0 C and 3000 C
decreases substantially as the cure temperature increases. That is, each cure at a higher
temperature progressively removes more solvent from the film. There is no residual
solvent in the film cured at 3000 C, Figure 3.2(e).
The temperature at which the solvent boils off during thermogravimetric analysis
also increases with the increased cure temperature. This can be more clearly seen in the
plots of the first derivative of weight loss (with respect to scan time) in Figure 3.2. The
temperature at which maximum solvent vaporization occurs increases from about 200 0 C to
about 3000 C as the cure temperature of the film is increased from room temperature to
200 0C.
The observed distribution of solvent removal temperatures, ranging from under
0
150 C up to about 3000 C, is most like the result of polymer-solvent binding. Loosely
bound solvent molecules readily evaporate below the boiling point of the solvent, while
tightly bound solvent molecules require much higher temperatures in order to evaporate.
As the material is cured, loosely bound species vaporize first, and the distribution of
removal temperatures of residual solvent moves to higher temperatures.
In Figure. 3.3, thermogravimetric scans of unirradiated and UV irradiated film
cured at 100 0 C are compared. Not only is there less solvent in the UV cured film, but also
the volatilization of the residual solvent occurs at higher temperatures after UV exposure. It
is thus apparent that the UV curing, by crosslinking the polymer chains, affects the solvent
removal dynamics of the thick film.
As Figures 3.2 and 3.3 show, at around 400 0 C, Probimide® 412 begins to
decompose. The decomposition temperature is arbitrarily defined as the temperature at
which the time derivative of weight loss equals 3% of the polymer per minute (scanning at
100C/minute). For our Probimide® 412 samples, decompostion is around 5200 C in
flowing air. Other authors [10,11] previously report a decomposition temperature of 5270 C
in flowing nitrogen.
110
olnu!w / % Iq6!eAA
0
Qt
L.
E
I-
% LIB!eM
Figure 3.2. Thermogravimetric weight loss and derivative of weight loss vs. temperature
for Probimide® 412 thick fims with highest cure temperature: (a) room temperature,
111
elnu!w / % lbIq!eM
o
Ln
lil
(D
0
**MO
o
LM
E
Lu
E
I-
0O
u,
r
%Flq.!eM
Figure 3.2 continued. (b) 100 0C
112
elnu!u / %l1I!oM
0
0
tO
U')
0o
L..
EI
G)
E
F-
% L16!0M
0
Figure 3.2. continued (c) 140 C
113
elnu!iw / % q6!eM
LO
~
C14
6
~_~_ ___ L ~~IIC -_ ~~I_ -----·I~----------···C·ll
·-- · · ·
CD
6
0o
Q
L
CIO
CL
E
i-c0
% q6!1eM
Figure 3.2. continuted. (d) 2000 C. Derivative curves plotted with two different full scale
values, curve (1), 10%/minute, and curve (2), 0.2%/minute.
114
aenu!W / %
%LB!aM
o
0
(D
0
LO
tO
0
4.'
a
E
I-
% ,l61!eM
Figure 3.2. continued. (e) 3000 C.
115
0
0
(D
0
V0
0
0
=3
-I
0
CL
0E
E
0
0
0
t,
0
LO
M•
ua3Jaad
0
tw
.i4l!aM
Figure 3.3. Thermogravimetric weight loss vs. temperature for Probimide® 412 thick
films baked at 100 0C, UV irradiated or unirradiated.
116
3.3.3. Dynamic Mechanical Analysis
The evolution of mechanical properties during the cure of Probimide® 412 bulk
films was investigated by dynamic mechanical analysis. The dynamic mechanical spectra at
1 Hz for Probimide® 412 samples progressively treated at 100 0C, 2000 C, 3000 C, and
4000 C are shown in Figure 3.4(a-d) respectively. Young's modulus and dissipation factor,
tan8, are shown as functions of temperature. For Figure 3.4, the 3000 C and 400 0 C cures
were performed in nitrogen. The DMA scans show both glass transition and secondary
transition behavior.
The room temperature Young's modulus does not appear to be dependent upon cure
conditions. The glassy moduli of our films are about 2.2-2.5 GPa. Other authors report a
modulus of 2.6-2.9 GPa for hard baked Probimide® 400 series films [10,13,14].
For the samples cured in air at 100 0C, Figure 3.4(a), and at 200 0C, Figure 3.4(b),
Young's modulus decreases by about an order of magnitude at 375 0 C. Tan5 shows a
strong maximum at 375 0C, which is the glass transition temperature. Several broad and
weak maxima occur near 100 0C and 250 0C.
In the film cured in nitrogen at 300 0C, Figure 3.4(c), the glassy modulus decreases
slightly at 2800 C and sharply at 375 0 C. Above 4250 C, the modulus exhibits an upturn due
to the thermal induction of covalent crosslinks. The dissipation factor, tan5, shows a
strong glass transition at 3750 C and a weaker, but distinct, secondary relaxation at 2800 C.
In the film cured at 4000 C in nitrogen, Figure 3.4(d), above room temperature, the
modulus declines with increasing temperature. There is a secondary relaxation at 280 0C,
and a broad relaxation above the 375 0C glass transition temperature seen in the other
samples. TanS shows several broad steps. There is no clear glass transition peak seen in
this sample.
The glass transition temperatures of all of the samples are near 375 0C. However,
while the samples treated at 1000 C, 200 0 C, and 300 0 C have sharp glass transitions at
375 0 C, the sample treated at 4000 C has a broad relaxation shifted to higher temperatures.
Ree et al. [14] used dynamic mechanical analysis to show that thermal cures from 3500 C400 0C induce chemical crosslinks in Probimide 412. Our results confirm that the 400 0C
treatment in nitrogen causes the polymer to crosslink, reinforcing the rubbery modulus and
shifting the softening transition to higher temperatures. We also show that the thermal
crosslinking does not occur in the films cured at 300 0 C and below.
117
Secondary relaxation also differs among the samples. Ree et al. [14] observed
broad and weak P relaxation at low temperatures and attributed the phenomenon to the
relaxation of phenyl moieties or to absorbed moisture. Our samples cured at 1000 C and
2000 C have weak and broad secondary relaxations. On the other hand, the samples cured
at 3000 C and 4000 C, which have been shown by TGA to be free of solvent, have well
defined relaxations at 280 0 C.
Ultraviolet irradiation crosslinks the Probimide ® 412 thick films, affecting its
relaxation behavior. In Figure 3.5, unirradiated and UV irradiated samples initially cured at
100 0 C are compared. The crosslinking causes an elevation of the glass transition
temperature by a few degrees, reinforces the rubbery modulus, and broadens the tan8 peak.
Ree et al. [14] observed that UV exposure reinforced the rubbery modulus in samples that
had been subsequently hard baked at 350 0 C in nitrogen. They found, however, that
thermally induced crosslinks created by subsequent curing at 4000 C dominated over the UV
induced crosslinks such that no effect of the UV could be observed.
The polymer also crosslinks when it is cured at 3000 C in an oxygen-containing
environment. This is demonstrated in Figure 3.6, where the mechanical properties of
samples cured at 3000 C in air and nitrogen are compared. While the sample baked in
nitrogen has a sharp glass transition at 375 0C, the sample baked in air has a broad glass
transition and a highly reinforced rubbery modulus. It is apparent that the 300 0C cure in air
has a similar effect on the mechanical properties as does the 400 0C cure in nitrogen. A
4000 C cure in air, however, severely oxidizes and degrades the specimen.
118
9uel
O
C)
aL.a
C)
(D
W
E
I
I
(ed) snlnpofl
I
I
I
I
s,BunoA o-1
Figure 3.4. 1 Hz dynamic Young's modulus and loss factor, tans, vs. temperature for
ProbimideO 412 thick films with highest cure temperature: (a) 100 0 C,
119
S9ue
0
c0
0S
- - - -
co
0
-
----
I
I
CD
0
--
--
0
-
I
---
-
-CD
.C
,#*a
0Lo
0
00
CD_
o
0
--
-l-
I
1~-·1111~·1)~---··
---
--
I
· I~-·---~-~----·--
(ed) snlnpol/ s,SunolA
Figure 3.4. continued. (b) 200 0 C,
I
o-i
--
I
E
CD
120
2Uel
0
6
dI
65
SI
0
d
6
1I-
v-
ew
0
o
0O
0
0
__~
I
,-)
-~
,.
~
___,
-
C
co0
(ed) snlinpoj
I
I
1
o-(0
,
s,bunoA to'1j
Figure 3.4. continued. (c) 300 0 C (nitrogen),
-
----
·----
oCDJ
E
0
!-.
121
0
--
I --
-
I~---
I
--
.
"
0
0
0
I'K
E
CH
/
_ 11_1
__ I_ I~ -_~_
--~------~-T-LIC
I --.
mI
ii
(ed) snlnpo/V s,6unoA 6o-1
Figure 3.4. continued (d)400 0C (nitrogen).
-s-
122
@ uo/
U0j
0o
tO
a\J
00
)
)
)
)
C.)
o
::3
-4-0)
1)
k-..;
f
w/
%yJ
( Od) snlnpoqi s Dunok bo
Figure 3.5. 1 Hz dynamic Young's modulus and loss factor, tan6, vs. temperature for
Probimide® 412 thick films cured at 100 0 C, UV irradiated (dashed line) or unirradiated
(solid line).
123
u i
0
0
LO
0
0
0
O
0
00
0
tO
0
0
,;I
0
0
0
0
E)
0
0o0
E
t)
HL
0
0
0
LOr"11%
0)
(od ) snlnpolAy s,bunol ,
o07
Figure 3.6. 1 Hz dynamic Young's modulus and loss factor, tan8, vs. temperature for
ProbimideO 412 thick films cured at 300 0 C, in air (dashed line) or in nitrogen (solid line).
124
3.3.4. Index of Refraction
The index of refraction of the polyimide changes as it is cured. Measurements of
the refractive indices (. = 632.8 nm) of spin coated Probimide ® 412 films are shown in
Figure 3.7. Average indices obtained for bulk films were quite similar to those obtained
for the spin coated films, and thus, only the results for the thin films are presented. The
average index is about 1.61-1.62 for samples treated at 1000 C and 2000 C in air, and for the
sample treated at 3000 C in nitrogen. However, the index of refraction increases after the
films are treated at 3000C in air or at 4000 C in nitrogen. We attribute the large increase in
refractive index to thermally induced crosslinking which takes place at 3000 C in air, but at
4000 C in nitrogen. These results are consistent with the observed color changes and with
changes in the dynamic mechanical relaxation spectra. Previous work [8] reports an inplane index (%= 1.06 gtm) for Probimide® 400 series films of approximately 1.61,
although the authors did not observe an increase in refractive index upon hard curing to
400 0C.
Ultraviolet curing also affects the optical properties of the Probimide® 412 film.
Figure 3.8 shows the index of refraction of 100 0 C softbaked samples as a function of UV
irradiation time. Refractive index increases with UV exposure as a result of crosslinking.
Gelation occurs rapidly at first, but slows with larger exposure times.
3.3.5. Fourier Transform Infrared Spectroscopy
We used FTIR spectroscopy to determine how thermal and UV curing changes the
chemical structure of the polyimide. Figures 3.9, 3.10, and 3.11 show the effects of
curing conditions on the FTIR spectra of Probimide® 412. Figure 3.9(a-d) demonstrates
the effect of UV exposure, while Figures 3.9 and 3.10 show the effects of thermal cures.
Figures 3.9 and 3.11 are scaled to three times the height of the N-C imide peak at 1370
cm- 1. Figure 3.10, which shows only the 2000-4000 cm- 1 region, is scaled to one-third
the height of the 1370 cm- 1 peak. Table 3.2 lists the main absorption bands observed and
the bond motion tentatively assigned to the vibrational frequency.
Several IR peak are of particular interest. Polyimides are well known to absorb in
the infrared strongly at 1720 cm -1 and weakly at 1770 cm - 1 due to carbonyl absorption
[15]. According to Higuchi et al. [16] weak absorption can be observed at 1680 cm -1 due
to benzophenone carbonyl. This was based upon the observation that only benzophenonecontaining polyimides absorb at this frequency [17]. The broad absorption regions
observed near 3000 cm -1 and 3500 cm- 1 are also of interest. The absorption near 3000
125
1.65-
1.64-
1.63 -
7/V"
1.62 1.61-
A-
/$0
I
~
1 A
•-
0
-
ml-
le
-
U
Y
0
U
U
Cure Conditions
Figure 3.7. Indices of refraction of spin coated Probimide® 412 films as a function of
highest cure temperature.
126
1.625 I
S
1.620-
6
0
I
1.615
-
120
180
240
300
UV Cure Time (minutes)
Figure 3.8. Indices of refraction of spin coated Probimide® 412 films as a function of UV
exposure time.
127
cm -1 is attributed to a series of overlapping hydrocarbon stretching bands [18,19], while
the broad absorption near 3500 cm- 1 is attributed to the stretching of highly hydrogen
bonded hydroxyl groups [18,19].
UV Effects
The chemical mechanism of UV crosslinking in inherently photosensitive
polyimides has been studied [16,20-22]. UV crosslinking occurs as follows. Hydrogen is
first abstracted from an alkyl group (see Scheme I). The resulting free radical attacks the
benzophenone carbonyl group (in preference to the imide ring carbonyls), altering the bond
to a carbon-hydroxyl group (see Scheme II).
R
R
H
'C
'C'
R
I:
+H
hv
o
II:
+ H
R
--
'C'
R
R
R
Probimide® 412 films spun on KBr were softbaked at 100 0C for 15 minutes and
cured under ultraviolet. The FTIR spectra at different UV exposure times are shown in
Figure 3.9(a-d). Significant absorption band changes that occur with increased UV
exposure are observed in Figure 9. First, broad absorption near 3500 cm-1 systematically
increases with UV exposure, indicative of an increase in hydroxyl groups. Second, the
broad absorption near 3000 cm-1 declines, indicative of a decrease in hydrocarbon bonds.
Third, the peak at 1680 cm-1 due to the benzophenone carbonyl group [16] systematically
recedes into a shoulder. This is accompanied by a decline in carbonyl absorption at 1720
128
cm -1. We also note the peak at 1770 cm- 1 due to the imide ring carbonyl bonds does not
seem to be affected.
Thermal Effects
The effects of thermal cures on infrared absorption are shown in Figure 3.10
(2000-4000 cm- 1 range) and Figure 3.11 (400-2000 cm -1 range). Changes in the FTIR
spectra occur when the polymer is cured at 300 0C in air, Figure 3.10(b,c) and Figure
3.11(b,c), or at 400 0 C in nitrogen, Figure 3.10(d) and Figure 3.11(d). Under these
conditions, the IR spectra from 2000 to 4000 cm- 1 in Figure 3.10 show a decrease in
absorbance from C-H bonds (near 3000 cm- 1) and an increase in absorbance from O-H
groups (near 3500 cm- 1). This suggests that the mechanism of thermally induced
crosslinking may be similar to that of the UV induced crosslinking. The IR absorption
spectra from 400 to 2000 cm -1 in Figure 3.11 indicate that, as in the UV exposed samples,
the carbonyl peak at 1720 cmn1 declines and the benzophenone carbonyl peak at 1680 cm- 1
becomes slightly flatter. We also notice that there is general broadening of all peaks across
the finger print region of the spectra in the thermally crosslinked samples. Virtually all of
the peaks appear wider and less resolved under conditions of thermal crosslinking. Thus,
there are probably many random, nonspecific degradation and crosslinking reactions
simultaneously taking place during the thermal cure. The aliphatic-group attack upon
carbonyl may be one of many reactions.
3.4. Conclusions
Cures above 2000 C are required to remove solvent from thick films of Probimide®
412. However, ultraviolet curing also drives out solvent. Regardless of cure treatment,
however, the films contain about 1%absorbed water. Thermogravimetric analysis reveals
a decomposition temperature around 5200 C.
Probimide® 412 undergoes glass transition softening at about 3750 C and a
secondary relaxation at about 280 0 C. Although residual solvent does not appear to affect
the primary glass transition, it may induce secondary relaxation at lower temperatures.
Ultraviolet curing at X = 364 nm crosslinks the polymer. The crosslinks reinforce
the rubbery modulus and broaden the distribution of relaxation times. The UV exposure
yellows the polymer and increases its refractive index. The crosslinking mechanism is
129
Table 3.2. IR absorption regions of Probimide ® 412.
Wavenumber (cm -1)
3600-3400
3000-2800
1770
1720
1680
1610
1600
1490
1420
1370
1300
1240
1100
850
710
t see references 15-19
Tentativ e Assignmentt
O-H
C-H
C=4D(imide)
C=O (imide and benzophenone)
C=O (b4
enzophenone)
C-C(phenyl)
C-C(phenyl)
C-C(phenyl)
C-C (phenyl)
N-(C(imide)
C-C-C (t,enzophenone)
C-F
N-(
C(imide)
phenyl substitution
phenyl substitution
s = strong, m = medium, w = weak
Strength*
w
w
m
s
w
w
w
m
m
s
m
m
m
w
m
130
0o
o
0
o00
E
0
C',~
0
0)
(sl!un AJejl!qJe) eoueqJosqv e!leAIeH
Figure 3.9. Relative IR absorbance vs. wavenumber for spin coated Probimide® 412
films softbaked 15 minutes at 100 0 C and exposed to UV. Arrows point to carbonyl (1680,
1720 cm-1), hydrocarbon (2800-3000 cm-1) and hydroxyl (3400-3600 cm -1) bands. (a) 0
minutes (no irradiation).
131
o
o
C0
O
(sl!un A~Jejl!qje) aueqjosqV aA!lelae
Figure 3.9 continued. (b)2 minutes,
E
O
T
132
I
*L4
O
0
C-
E
Cl
-0
-o
--
--
-
(sp~un rijejl!qje) a~ueqjosqVO·
Figure 3.9 continued. (c) 10 minutes,
--
C3
a0
0
J------
(Cs
ljejGHt
133
E0
0
o
C)
CD
(
(spun Ajejl~qle) e3ueqjosqV eanejlaj
Figure 3.9 continued. (d) 50 minutes.
134
a)
0
C.
cu
0
_,
C_
Cu
0.
a)
FL'
P
a)
U)
>
C-.
U,
a)
.0=
Cu
dO
Cu,
~Cu
4000
3500
3000
2500
2000
Wavenumber (1/cm)
Figure 3.10. Relative IR absorbance vs. wavenumber (2000-4000 cm- 1 range) for spin
coated Probimide® 412 films with highest cure temperature treatment: (a) 300 0 C (3 hours
in nitrogen), (b) 300 0 C (1 hour in air), (c) 300 0 C (3 hours in air), (d) 4000 C (1 hour in
nitrogen).
135
0
0
Lt)
o
0
0
r
E
0
Lu
.0
E
c~.
00
0,
(sl!un AJeja!qJe) eoueqJosqv e!jeletj
Figure 3.11. Relative IR absorbance vs. wavenumber (400-2000 cm -1 range) for spin
coated Probimide® 412 films with highest cure temperature treatment: (a) 300 0C (3 hours
in nitrogen), {arrows point to carbonyl bands at 1680 cm -1 and 1720 cm- 1)
136
o
LOl
0
0o
gE,
r)
42
0
igL
C
(spun AjeJlqJe)
9ueqJ~osqv
(s!un A~e.Il~q~e) eoueq~osqy
Figure 3.11. continued. (b) 3000 C (1 hour in air),
lcE
ar!elaeie
137
=
--
c
-
I---~----~
o
O
*J
0
o
0O
E
0)
-o
1m
-
-=-
-
-
------------
--
---
(sl!un Ajejl~qje) 9oueqjosqV 8A!jejajl
Figure 3.11. continued. (c) 300 0 C (3 hours in air)
o
0
(q4
138
II
I
__1 _·_1_1____1__________
-o
O
-o
O
U
O
L.
00
E
Ca
-o
Lr
_
----- --- --
-~II
(sl!un A3ed!q.e) eoueqJosqur
-
en!ie le).
Figure 3.11. continued. (d) 4000 C (1 hour in nitrogen).
-I
139
highly specific and occurs by hydrogen abstraction and attack of the free radical at the
benzophenone carbonyl.
The material also undergoes substantial thermally induced crosslinking which is
especially favored in an oxygen containing environment. We found extensive crosslinking
to occur when films are baked at 4000 C in nitrogen or at 3000 C in air. A nitrogen
environment appears to inhibit crosslinking at 300 0C. The thermal crosslinking process
makes the polymer more brittle, reinforces its rubbery modulus, and broadens its
distribution of relaxation times. Also, it gives the material a deep yellow to brown color
and increases the index of refraction by approximately 20%. Although one possible
crosslinking mechanism may be similar to that of the UV process, the thermally induced
crosslinking is probably not limited to one mechanism.
3.5. References
1.
H. Hiramoto, Mat. Res. Soc. Symp. Proc., 167, 87 (1990).
2.
R. Rubner, H. Ahne, E. Kuhn, and G. Kolodziej, Photo. Sci. Eng., 23(5), 303
(1979).
3.
J. Pfeifer and 0. Rhode, in Recent Advances in Polyimide Science and Technology,
W.D. Weber and M.R. Gupta, Eds., Society of Plastic Engineers, New York, 1987,
p. 336.
4.
0. Rhode, P. Smolka, P. Falcigno, and J. Pfeifer, Polym. Eng. Sci., 32(21), 1623
(1992).
5.
Japanese patent #5034699, A (1993).
6.
J. Cognard, Mol. Cryst. Liq. Cryst. Suppl. Ser., Suppl. 1 (1982).
7. K.K. Chakravorty and C.P. Chien, SPIE Int. Conf. on Adv. in Intercon. and
Packaging, 1389, 559 (1990).
8.
C.P. Chien and K.K. Chakravorty, SPIE Opical. Thin Films III: New Developments,
1323, 338 (1990).
140
9.
R. Mo, T. Maw, A. Roza, K. Stefanisko, and R. Hopla, Proc. 7th Int. IEEE VLSI
Multilevel Intercon. Conf., 390 (1990).
10. T. Maw and R. Hopla, MRS Symp. Proc., 203, 71 (1991).
11. T. Maw and R. Hopla, MRS Symp. Proc., 227, 205 (1991).
12. T. Maw, M. Masola, and R. Hopla, Polym. Mat. Sci. Eng., 66, 247 (1992).
13. M. Ree and K. Chen, Polym. Prep., 31(2), 594 (1990).
14. M. Ree, K. Chen, and G. Czornyj, Polym. Eng. Sci., 32(14), 924 (1992).
15. H. Ishida, S. Wellinghoff, E. Baer, and J. Koenig, Macromol., 13, 826 (1980).
16. H. Higuchi, T. Yamashita, K. Horie, and I. Mita, Chem. Mater., 3, 188 (1991).
17. R. Dine-Hart and W. Wright, Die Makromolekulare Chemie, 143, 189 (1971).
18. R. Silverstein and G. Bassler, Spectrometric Identification of Organic Compounds,
John Wiley & Sons, New York, 1967.
19. C. Pouchert, The Aldrich Library of Infrared Spectra Edition III, Aldrich Chemical
Company, Milwaukee, WI 1981.
20. J. Scaiano, A. Becknell, and R. Small, J. of Photochem. Photobio. A, 44, 99
(1988).
21. A. Lin, V. Sastri, G. Tesoro, A. Reiser, and R. Eachus, Macromol., 21(4), 1165
(1988).
22. J. Scaiano, J. Netto-Ferreira, A. Becknell, and R. Small, Polym. Eng. Sci, 29(14),
942 (1989).
141
Chapter 4. Effect of Cure Conditions on Probimide® 32
Polyamide-imide
4.1. Introduction
Polyimides, as discussed in the previous chapters, are important members of the
class of high temperature heterocyclic polymer materials [1-3]. They are well known for
their outstanding thermal stability, mechanical and electrical properties, and chemical
resisitance. Polyimides are used in electronic, display, and aerospace applications.
Polyamide-imides [4] are related polymers whose repeat units contain both an imide
ring linkage and an amide linkage. Polyamide-imides are less expensive than polyimides,
readily processed from solutions, and still have outstanding properties. There is expanding
interest in polyamide-imides as engineering plastics [4], as materials for molecular
composites [5,6], and as liquid crystal display (LCD) alignment films [7].
In Chapter 1, it is explained that polyimides generally require high temperature
treatments which are necessary both for ring closure of the polyamic acid precursor, and
for the removal of high boiling point solvents. Studies show [8-16] that the chemical and
physical properties of polyamic acid precursor systems change substantially as they are
cured, primarily because of the chemical conversion from polyamic acid to polyimide.
Unlike most polyimides, polyamide-imides are available in solutions of their fully
converted (i.e., fully imidized) form. The fully imidized polyamide-imides systems do not
require post-processing heat treatment to effect closure of the imide ring.
It is important that the outstanding features, as well as the limitations, of polyamideimides be well understood. It is furthermore crucial to understand these materials from a
processing-structure-properties perspective. Since reducing the cure temperature of
polyimides may reduce processing cost and time, as well as allow their compatibility with
inexpensive low temperature materials, it is interesting to learn how these systems respond
to differing thermal treatments. Thus, this work explores the effects of cure temperature
and environment on the properties and structure of a preimidized polyamide-imide system.
Our goal is to learn how the pre-converted polymer changes physically and chemically
during its cure treatment. We report here the processing-structure-property relationships in
Probimide® 32 polyaimide-imide as a part of an ongoing evaluation of the material for
liquid crystal alignment applications.
142
4.2. Experimental Section
4.2.1. Polymer film processing
We studied the material Probimide® 32, a fully imidized polyamide-imide resin
produced by OCG Microelectronic Materials, Inc. The polymer is provided in an
NMP/xylene solution with 21% solids content by weight. The material has the following
chemical repeat unit:
+
n
Bulk films were formed by the technique described in Section 3.2.1 and depicted in
Figure 3.1. The Probimide® 32 solution was dropped onto a glass plate and wiped with a
metal doctor blade with a 1 mm gap. The wet films were dried to a tack free condition
under a dry air current at room temperature overnight. The films were then progressively
cured at 100 0C for six hours, then 2000 C for three hours, and then 3000 C for three hours.
Thermal cures of 100 0C and 2000 C were performed in air with the polymer on glass, while
the 3000 C cures (which we will call "hard cures") were performed on free-standing film in
either air or nitrogen environments. Film thicknesses were on the order of 100p.m. All
films were amorphous, as the samples exhibited no Bragg scattering peaks when examined
by wide angle X-ray scattering.
Thin films were formed by spin casting the undiluted Probimide® 32 solution on
glass or KBr at 4000 RPM for 30 seconds. The samples were immediately softbaked at
100 0 C for fifteen minutes in air. The samples appeared to be dry after softcuring. The
samples then progressively underwent one hour cures at 2000 C and then 3000 C. The 200 0C
cures were performed in air, while the 3000 C cures were performed either in air or
nitrogen.
143
4.2.2. Measurement techniques
A Seiko TG/DTA320 was used for thermogravimetric analysis of the bulk films.
Samples were scanned from room temperature to 5750 C at 100C/minute in flowing air
(300ml/minute).
Dynamic mechanical analysis was performed in tension mode on bulk films using a
Seiko DMS200. Samples were scanned under nitrogen at 20C/minute from room
temperature to the temperature above Tg at which breakage occurred. Frequencies 1, 2, 5,
10, and 20 Hz were used.
Index of refraction at X = 632.8 nm was measured using a Metricon PC-2000 prism
coupler. Refractive index was measured by determination of the critical angle. Both inplane nTE and out-of-plane nTM indices were measured. Average refractive index navg was
calculated as:
navg = (1/3) (2nTE + nTM).
Refractive index measurements were taken for the free standing films and the thin films
spin coated on glass slides.
Fourier transform infrared spectroscopy was performed using a Nicolet 510P.
FTIR spectra were taken for the Probimide® 32 thin films spin coated on KBr.
Differential scanning calorimetry (DSC) measurements were made for bulk films
using a Perkin Elmer DSC-7 calibrated to zinc and indium standards. The scan rate was
200 C per minute.
4.3. Results and Discussion
4.3.1. Coloration and Brittleness
The changes in coloration and brittleness in Probimide 32 thick films that occur
with subsequent cures are summarized in Table 4.1. The films were considered "brittle" if
they were easily broken upon flexing. Probimide® 32 becomes progressively darker as it
is cured to higher temperatures, and it is darkest when cured at 3000 C in air. The polymer
is brittle after it is cured at only 100 0 C. However, the brittleness recedes when it is cured to
2000C. The film then becomes brittle if cured at 300 0C in air, but does not become brittle if
cured at 3000 C in nitrogen. The brittleness and dark coloration of the air cured, hard baked
film suggests that abundant chemical crosslinks have formed.
(1)
144
Table 1. Color and Brittleness in Films Cured at Progressively Higher Temperatures.
Temperature
Environment
Color
Brittleness
100 0C
200 0C
300 0C
3000C
air
air
air
N2
yellow
brown
dark, reddish brown
brown
very brittle
flexible
brittle
flexible
4.3.2. Thermogravimetric Analysis
Thermogravimetric spectra of Probimide® 32 bulk films chart the removal of
solvent and water from the films, as well as indicate decomposition of the polymer. Our
primary goal is to observe solvent content as a function of cure conditions. We
demonstrate here how solvent is progressively removed from the bulk polymer films as
they are cured at 100 0C, 2000 C, and 300 0C.
Polyimide Solvents
Polyimide solvents typically have very high boiling points. N-methylpyrrolidone
boils at 202 0 C, while xylene boils at 138 0C-1440 C. However, while solvents unbound to
polymer typically evaporate at temperatures below their boiling points, solvents bound to
polymer may require higher temperatures in order to vaporize. In the 100C/minute
thermogravimetric scan of Fisher Scientific N-methylpyrrolidone in Figure 4.1, all solvent
has evaporated by about 190 0C.
Probimnide32
In Figure 4.2(a-d), thermogravimetric and derivative scans of Probimide® 32
treated at room temperature, 100 0 C, 2000 C, and 300 0 C, respectively, are compared. The
scans depict the loss of water at 800 C, loss of residual solvent at about 2000 C, and
decomposition of the polymer above 3500 C. The films lose about 3% of their initial weight
below 150 0C due to water removal. All samples treated at or below 2000 C have a large loss
in weight in the 150 0 C to 350 0 C region due to vaporization of residual solvent. For the
derivative curves, the higher the cure temperature, the higher the temperature at which
maximum rate of weight loss of solvent occurs. While the rate of weight loss is maximal at
145
elnu!uw / %jq6!eM
0
0
oL.
I-E
0
Figure 4.1. Weight loss and derivative of weight loss vs. temperature for N-methyl
pyrrolidone (NMP).
146
elnu!W / % 16I!eM
0
C)
L
3
EI
CL
0.
E
C)
% I6G!aM
Figure 4.2. Weight loss and derivative of weight loss vs. temperature for Probimide® 32
thick films: a.) dried under an air current at room temperature,
147
elnu!W /% lqSi!eM
0
(0
CDj
C)
I-
E
'mm
0
a
% tI5!eM
Figure 4.2. continued. b.) then baked 6 hours at 100 0C,
148
elnuw/%
Lq6i!M
0
(0
lq
LO)
0
%%O
-e
LM
CLI
E
)I-
%0/;tt6!eM
/0 61RMA
a1~V
Figure 4.2. continued. c.) then 3 hours at 200 0 C,
149
elnu!
/ % lqI!eM
o
0
0D
qzr
O
E
)-
0
CO
% 1L6!eM
Figure 4.2. continued. d.) then 3 hours at 300 0 C.
0
I%-
150
around 200 0 C in the room temperature cured sample, the rate of weight loss is maximal at
almost 300 0 C in the 2000 C cured sample. The peak height decreases as the cure
temperature increases. In the 150 0C to 350 0C region, the room temperature cured sample
(Figure 4.2(a)) has the largest weight loss, the 100 0C cured sample less (Figure 4.2(b)),
and the 200 0C cured sample even less (Figure 4.2(c)). Thus, each cure at a progressively
higher temperature removes more solvent from the film. There is apparently no residual
solvent remaining in the 300 0C cured film (Figure 4.2(d)). The weight loss curves shows a
plateau until the temperature exceeds 4000 C, at which point film degradation begins.
As also shown in the previous study of Probimide® 412, there is a distribution of
solvent removal temperatures ranging from below 150 0C to above 350 0 C. The 32 films,
however, contained more solvent than the 412 films of Chapter 3. The distribution, hence,
is even more apparent. Solvent molecules removed at or below the boiling point of the
solvent represent loosely bound species that readily vaporize, while molecules that are
removed at temperatures far above the boiling point represent species that are physically
bound to the polymer. Thus, as the material is cured, low boiling species are removed, and
the distribution of removal temperatures of residual solvent species is pushed to higher
temperatures.
Cure time also affects the residual solvent in the polymer film. Figure 4.3 depicts
thermogravimetric scans of Probimide® 32 treated for 1, 6, and 24 hours at 100 0C. As
cure time increases, the amount of residual solvent decreases. There is also a small shift in
the distribution of solvent removal temperatures between 1 and 6 hours and a larger shift
after 24 hours of cure.
As Figures 4.2 and 4.3 show, at around 450 0C, decomposition of the polymer
begins, as indicated by a large loss in weight. If we arbitrarily define the decomposition
temperature as the temperature above 350 0 C at which the time derivative of weight loss
equals 3% per minute (scanning at 100C/minute), the decomposition temperature of
Probimide® 32 is about 550 0 C in flowing air. The manufacturer reports a decomposition
temperature of 494 0 C [17] though they do not report the scan conditions.
151
O
(0
C)
O
t)
o
0
O
O 4--
E
O
tO
0
0
O
0
C
0
O
(1
0O
lua3aJd 14q!aM
Figure 4.3. Weight loss and derivative of weight loss vs. temperature for Probimide ® 32
thick films baked at 100 0C for 1, 6, and 24 hours.
152
4.3.3. Dynamic Mechanical Analysis
The dynamic mechanical relaxation of Probimide® 32 films provides a wealth of
property and structure information. DMA data demonstrate how the glassy regions, the
relaxation regions, and the rubbery regions are affected during the cure and how they are
dependent upon cure environment. The changing dynamic mechanical spectra can be
attributed to changing solvent content and to changing degrees of crosslinking.
The DMA spectra for Probimide® 32 are shown in Figure 4.4(a-d) for samples
cured at 1000C, 2000C, 3000C in air, and 3000C in nitrogen, respectively. Young's
modulus E' and loss factor tan8 are shown as functions of temperature. For brevity, only
measurements at 1Hz are shown.
The glassy Young's modulus at 250C is affected by the cure conditions of the
polymer film. The glassy modulus of film treated at 1000C and 2000C (Figure 4.4(a,b)) is
1.9-2.0 GPa. The glassy modulus of Probimide® 32 film treated at 3000C in nitrogen
(Figure 4.4(d)) is somewhat higher, 2.7 GPa, which agrees with reported values of the
modulus [17]. It appears that residual solvent in the films cured at 1000C and 2000C
plasticizes the films, lowering the glassy modulus. The measured glassy modulus of film
treated at 3000C in air (Figure 4.4(c)) is fairly low, 1.3 GPa. This is either because of
oxidative damage inflicted upon the polymer during the cure, or because of an inability to
make an accurate measurement due to the brittleness of the film.
In Figures 4.4(a,b), Young's modulus for samples cured at 1000C and 2000C drops
by about two orders of magnitude as a result of polymer relaxation. In the tan8 curves,
two relaxations are seen in these samples. The first relaxation in each scan occurs at about
the curing temperature, while the second relaxation occurs just below 3000C. The material
is highly plasticized by residual solvent, giving the material a low softening temperature.
Once the solvent is driven out, the material again relaxes at the glass transition temperature
of the unplasticized polymer.
In Figures 4.4(c,d), the samples treated at 3000 C, in air or in nitrogen, have only a
single relaxation. The glass transition relaxation is centered above 3000C in the air-treated
sample. The tan8 peak is shallow, suggesting the sample is highly crosslinked by the
3000 C cure in air. The sample treated under nitrogen at 3000C, on the other hand, has a
well defined glass transition relaxation at 3000C. It is thus apparent that the degree of
crosslinking in Probimide® 32 is substantially greater when hard cured in air than when
hard cured under flowing nitrogen.
The rubbery modulus region is also very different in the Probimide® 32 films. The
films cured at 1000C and 2000 C have extremely low rubbery moduli, the 2000C cured film
153
having a slightly greater rubbery modulus than the 100 0 C treated film. In the 300 0 C
nitrogen-treated material, the rubbery Young's modulus reaches a minimum value of 0.16
GPa shortly after relaxation. This rubbery modulus is about ten times greater than that
observed in the 100 0 C and 200 0 C treated films. Apparently, the 3000C cure in nitrogen not
only increases the glass transition temperature, but also provides reinforcement to the
rubbery state, probably as a result of backbone crosslinks that form during the hard cure.
The rubbery modulus of the film cured 300 0 C in nitrogen slowly increases with temperature
as the high temperatures induce more non-oxidative crosslinks in the polymer. In the film
cured at 3000 C in air, no rubbery region is clearly observed because gel network has
already formed before the polymer relaxes. Instead, the modulus slowly declines and does
not appear to flatten entirely before the sample breaks.
In a polyimide that is not preconverted, the modulus and glass transition
temperature increase as the polymer is cured. This is not surprising, as the conversion of
polyamic acid to polyimide increases the stiffness of the chemical backbone. This study
shows, however, that even in a fully converted system, residual solvent can also have a
marked effect on the mechanical properties of the material. The study also demonstrates
that the extent of interchain covalent bonding strongly impacts the mechanical behavior of
the polyamide-imide.
4.3.4. Index of Refraction
The optical properties of Probimide® 32 are also cure-dependent. They change
with cure temperature and cure environment. The indices of refraction of thin films of
Probimide® 32 spin coated on glass are presented in Figure 4.5(a,b). The average indices
of refraction of the bulk films were identical to those of the thin films.
Average indices of refraction of spin coated films are plotted against highest cure
temperature in air in Figure 4.5(a). There is a general increase in refractive index with
increasing cure temperature. These changes coincide with the observed changes in
coloration described in Table 1. The increase in index is associated with increased density
and/or electronic polarizability due to a combination of several factors: loss of solvent,
crosslinking, and increased interchain interaction.
Average indices of films cured at 300 0C are shown in Figure 4.5(b) as a function of
3000 C cure conditions. The films hard baked films under nitrogen have slightly lower
average refractive indices than do those treated in air. This suggests that the crosslinking
reaction at 3000 C is faster in air than in nitrogen.
154
2Uejl.
CO
o;
d
o
oo
0
0
0
a0
O
L.
o
(1)
E
0C
SZIL.
('•
0N
0
1Q)
0
(ed) sninpoins,
,Aor i
tuno
Figure 4.4. Young's modulus and dissipation factor, tan8, for Probimide® 32 thick films:
a.) cured 6 hours at 100 0C,
155
9ueA
o
6
d
d
6o6
do
6
0
0
CD0
0
rafCD,
M
C))
CD 0
L
0g
2
I-~Z
E
0
T
W)
Co
(ed) snlnpoi s,6 unoA
Figure 4.4. continued. b.) then 3 hours at 200 0 C,
QD
15o
tf%
156
0D
0U.
D
9-
Ir
·I,
+1
C*
CD
CDJ
om #
E
0
1
0
I
(8d) sfIfPOV
II
SUno
U
Bo
(ed) sninpoyy s,Bunol( 6o-I
Figure 4.4. continued. c.) then 3 hours at 300'C in either air or,
C
157
ue.l
VCO
ltft0Ok
a
U
~7
r;
I-,
0
0D
0
0
CD)
C)
E
1a
v
a,
v.
m
w
M
(ed) snlnpoN s, unoA Bo-1
Figure 4.4. continued. d.) nitrogen.
I ^
158
1
II
75E
1.70-
U
U
1.65-
U
1.60
100
200
300
Highest Cure Temperature (oC)
Figure 4.5. a.) Indices of refraction of spin coated Probimide ® 32 films vs. highest cure
temperature in air. Cure cycle was 15 minutes at 100 0 C, 1 hour at 2000 C, 1 hour at 3000 C.
159
1.75 ICI
1.70-
~
II_
~I_
~
I
,
//--
1.65-
/0
1.60 II
Nitrogen (1 hr.)
-*-
r
l4
|
Nitrogen (3 hrs.)
Cure Conditions
b.) Refractive indices for different cure conditions at 300 0 C.
Air (1 hr.)
I
160
4.3.5. Fourier Transform Infrared Spectroscopy
Fourier transform infrared spectroscopy is a powerful method of examining the
progress of a polyimide cure. This is especially true in polyamic acid precursor materials,
which undergo significant changes in their infrared spectra as they are converted to
polyimides. Our fully preconverted polyamide-imide, on the other hand, gives us an
interesting opportunity to observe the effects of solvent removal and oxidative crosslinking
in the absence of the amic acid to imide conversion. We demonstrate here how the FTIR
spectrum of Probimide® 32 evolves during curing. The study below suggests a technique
for monitoring the cure in fully converted polyimide and related systems.
Solvent Spectra
N-methylpyrrolidone (NMP) and xylene are the two solvents used in the
Probimide® 32 system. Their chemical structures are shown below:
CH3
I
CH3
I
F•I CH3
NMP
xylene
FTIR spectra of these compounds can be found in most infrared spectra atlases [18]. The
main IR absorption regions and our peak assignments are shown in Table 4.2.
The three chemicals in the Probimide® 32 system, the polyamide-imide polymer,
NMP, and xylene have a number of chemical units in common. For example, both the
polyamide-imide and NMP contain an amide unit, the polymer a secondary amide, and the
NMP a tertiary amide. The carbonyl bond of the amide group in both molecules
contributes an extremely strong infrared absorption band at about 1680 cm- 1. Also, the
xylene and the polyamide-imide contain aromatic bonds, which have a number of IR
absorption bands, which are listed in Table 4.2. One key spectral peak, due to carboncarbon skeletal stretching of the aromatic ring, appears at around 1500 cm-1. There can be
multiple bands in this region at slightly different absorption frequencies depending upon the
ring substituents. In addition, all three substances have carbon-hydrogen bonds. These
bonds typically have broad absorption in the locality of 3000 cm- 1.
161
Table 4.2. IR absorption regions of NMP and xylene.
Compound
Peak Location or
Approx. Region (cm - 1)
Tentative
Assignment?
Peak
Strength*
NMP
2800-3000
v(C- H)
s
1680
v(C = O)
s
1380-1500
6(C- H), 5(N-CH 3 )
s
1300
v(C-N)
s
1110
v(C -N)
m
980
3(ring)
m
2900-3100
v(C - H)
s
1380-1650
v(C = C)
s
1000-1050
aryl 8(C - H)
m
790, 760, 740, 690
8 ring substitution bands
s
xylene
see references 18,19
s = strong, m = medium, w = weak
162
Probimide 32 Spectra
Fourier transform infrared spectroscopy of spin coated thin films of Probimide® 32
on KBr, shown in Figures 4.6-4.9, revealed two processes that take place in the system
upon curing. The first process is the systematic removal of solvent. The second is
crosslinking and degradation at 300 0 C in air. Assignments of the main absorbance bands
[18,19] in the polyamide-imide polymer are shown in Table 4.3.
The presence and removal of residual solvent is apparent in the "finger print" region
of the FTIR scans. Figure 4.6(a-f) shows the 400-2000 cm- 1 region of Probimide® 32
samples treated for 10 minutes at 20 0C intervals between 100 0C and 200 0 C. As the sample
is cured from 100 0C to 200 0 C (Figure 4.6(a-f)), a number of peaks decline or disappear.
These changes are consistent with removal of solvent. The most important peak is
associated with the amide carbonyl bond, which is present in both the polyamide-imide and
the NMP solvent. The tertiary amide in the NMP and the secondary amide in the
polyamide-imide both absorb at about 1680 cm-1. This strong absorption peak gradually
declines in Figure 4.6 as NMP is removed from the polymer by treating it to progressively
higher temperatures.
Observation of the strong reflection at 1510 cm-1 due to aromatic skeletal stretching
also proves quite interesting. As the polymer is treated to higher temperatures, an
overlapping side peak systematically recedes. The best explanation for this occurrence is
that NMP or xylene, either of which could contribute to this side peak, is systematically
removed. When the solvent is eliminated, a shoulder to the peak at 1510 cm -1 remains,
reflecting the fact that there are two differing aromatic substitution environments in the
polyamide-imide.
As the polymer is cured, we also observe the gradual disappearance of a shoulder
peak near 1300 cm- 1, decline of a peak at around 1110 cm- 1, and disappearance of a small
peak at about 980 cm- 1. All three of these changes are consistent with the gradual removal
of NMP from the system.
We also see an increase in the band 1770 cm- 1 associated with in-phase stretching
of the imide ring carbonyl. However, there is no obvious explanation for this.
The 2000-4000 cm- 1 region is shown in Figure 4.7. Two marked changes are
observed. First, there is decline in absorption in a broad range around 3000 cm- 1. This
indicates a decrease hydrocarbon bonds, and thus, is consistent with the removal of
solvent. Second, there is gradual shifting of the N-H amide peak from about 3300 cm-1 to
almost 3400 cm-1. It is typical for the location of this band in amide compounds to exist at
163
Table 4.3. Partial list of IR absorbance bands in Probimide® 32 polyamide-imide.
Peak Location or
~
~
L~
Tentative
Approx. Region (cm- 1) --- I~-3100-3400 (broad) -- ~---
Assignmentt
v(N-H)
Peak
Strength*
3000-3100 (broad)
v(C-H)
1770
v(C=O) (imide)
w ---------- w
--------- m ~---
v(C=O) (imide)
-~--~------ 5 ------
1720
"---
v(C=O) (amide)
-I---
v(C=C) (phenyl)
L I-~..I
s-r
II
1680
~---
1600
--
1510
----·----
S
II
1410
c
m ---
--
1380
~I---
1320
~I~
720
_
~
v(C=C) (phenyl) -------~
-~I- v(C=C) (phenyl) I----~--.
v(N-C) (imide)
v(N-C) (amide)
-------
8 phenyl
substitution
_C
_
_
see references 18,19
s = strong, m = medium, w = weak
m --
5
_ ~
-LI~I~-·C--~-C--
m
164
a lower wavenumber in the solid state than in the solution state as a result of increased
hydrogen bonding [19]. A shift to a higher wavenumber as solvent is removed suggests
that the polymer forms stronger hydrogen bonds with the NMP than it does with itself.
In Figure 4.8(a-d), 400-2000 cm -1 FTIR scans region of Probimide® 32 treated at
100 0 C, 2000 C, and 3000 C in air, and 3000 C in nitrogen, respectively, are shown. The
effects of solvent removal described above are clearly seen in these scans as the films are
treated above 100 0C. Also, we note that the peak at 1770 cm-1 corresponding to in-phase
stretching of the imide carbonyl increases as the polymer is heated from 100 0 C to 2000 C,
but returns to its original height after treating at 3000 C in either air or nitrogen. Again, there
does not seem to be an obvious explanation for this.
The 2000-4000 cm- 1 region is shown in Figure 4.9. Reduced absorption near 3000
cm-1, as well as shifting of the N-H peak from 3300 to 3400 cm-1, both a result of solvent
removal, are again apparent as the material is treated above the softbake temperature.
Previously, we showed that a 300 0 C treatment in air initiated substantial chain
crosslinking, while the same thermal treatment in nitrogen did not. There are noticeable
differences the FTIR spectra of Probimide 32 films treated at 300 0 C in air, Figures 4.8(c)
and 4.9(c), and at 300 0 C in nitrogen, Figures 4.8(d) and 4.9(d). These changes suggest
possible mechanisms for the oxidative crosslinking reaction. The most apparent difference
is a reduction in the air-treated sample in C=C phenyl stretching near 1510 cm-1. This is
accompanied by a drop in C-H stretching near 3000 cm- 1. In fact, two distinct
hydrocarbon peaks near 2900 cm- 1 are nearly eliminated. The amide C=O peak at 1680
cm-1 appears to become nearly a shoulder of the imide C=O peak at 1720 cm- 1. Also, there
is an increase in the C=C phenyl vibration at 1600 cm- 1.
Kuroda and Mita [20] also observed a large drop in the 1510 cm-1 phenyl vibration,
accompanied by an increase in the 1600 cm- 1 phenyl vibration, in their studies of thermooxidative degradation of a polyimide. They interpreted the decline in the 1510 cm-1 peak as
the cleavage of 4-CH 2 bonds. The authors speculate that the increase in the vibration at
1600 cm- 1 might be associated with the formation of diphenyl crosslinks, as suggested by
Dine-Hart et al. [21,22], though Kuroda and Mita [20] do not suggest this to be a primary
path of crosslinking in their material.
Dine-Hart et al. [21,22], in their investigations of the oxidative curing of PMDAODA polyimide, concluded that the main crosslinking reaction appeared to be the
intermolecular coupling of phenyl rings in the diamine segments of the main chains. This
occurred by the formation of phenolic groups which subsequently react. The phenolic
bonds were formed either by direct dehydrogenation of the phenyl rings, as shown in
Scheme I, or by scissioning of aryl-ether bonds.
165
[1O]
OH
H
-HO20
Scheme I
The reduction in the 1510 cm - 1 phenyl vibration, increase in the 1600 cm- 1 phenyl
vibration, and reduction in C-H stretching near 3000 cm- 1 in the Probimide 32 polyamideimide might be associated with the formation of the crosslinks shown in Scheme I. The
diphenyl crosslinks must occur in the diamine portion of the polyamide-imide repeat unit,
as the 1510 cm- 1 vibration is associated with 1,4-C 6H4 groups [23], while the 1600 cm -1
band can be attributed to more highly substituted phenyl groups [23]. The inter-chain
crosslinks in Scheme I might occur in the conjunction with the intra-chain breakage of 1,4phenyl-phenyl bonds in the diamine, as shown in Scheme II. The radicals in Scheme II
may subsequently produce random bond reformations.
Scheme II
166
Broadbelt et al. [24] reviews the various reactions that may occur on the amide
linkage of an aromatic amide model compound as the substance undergoes elevated
temperature treatments. A reaction mechanism that would be consistent with an increase in
molecular weight would be a [1,3] sigmatropic shift (see Scheme III) followed by random
radical recombination reactions. It is also likely that many of the other reactions could
simultaneously take place, including various bond homolyses of the amide bond followed
by random radical recombinations which do not result in a net increase in molecular weight.
If the sigmatropic shift and random crosslinking at the amide bond were to occur in
the polyamide-imide during its 3000 C treatment in air, we would expect to see decreases in
amide C=O stretching at 1680 cm- 1 and in N-H stretching in the 3100-3400 cm-1 region.
These changes appear to have occurred in the FTIR spectra for the 3000 C air treatment in
Figures 4.8(c) and 4.9(c)).
0
I
"O-N'
0
+ OH0
+ OH
I
+ H20
Scheme III
The above data suggest that cure can be monitored in the polyamide-imide system
by FTIR. This could be an especially valuable tool if, for example, a new processing
scheme and cure cycle were to be tested for solvent removal effectiveness. The most
apparent change in the FTIR spectrum as solvent is eliminated is the decline of the amide
carbonyl peak at 1680 cm -1. In Figure 4.10, the peak height at 1680 cm - 1 relative to that at
1370 cm -1 is plotted against highest cure temperature for FTIR data shown in Figures 4.6
167
and 4.8. The ratio declines steadily with increasing cure temperature. Because the spin
coated films are thin, we expect the films to be nearly free of solvent after the one hour cure
at 2000C. The drop in peak height after the 3000C cure could be due to either further
removal of high boiling point solvent species or to non-oxidative crosslinking at the amide
carbonyl. The films appear to be solvent free when the relative absorption ratio is about
0.6-0.7.
4.3.6. Differential Scanning Calorimetry
The calorimetric relaxation response of Probimide @ 32 was studied as a function of
cure temperature and environment in order to gain further insight into the processes that
take place during curing. DSC scans of Probimide® 32 bulk films treated at 1000C, 200 0C,
and 3000C (air and nitrogen) are shown in Figure 4.11 (a-d), respectively. Several physical
phenomena can be observed in these scans: the vaporization of water, the vaporization of
solvent, and transition from the glassy to rubbery state.
Vaporization of water contained by the polymer, also observed in the TGA scans in
Figures 4.2 and 4.3, is seen in all DSC scans in Figure 4.11. It appears as a broad
endothermic response near 1000C.
Vaporization of residual solvent is observed in the 1000C treated sample, Figure
4.10(a), as indicated by the large endothermic response seen in the scan between 1500C and
300 0C. It is possible that the spikes observed are a consequence of instrument noise caused
by the violent vaporization occurring in the encapsulated sample. Any glass transitions that
might have occurred in the 1000C treated sample are hidden by the strong vaporization
response. Solvent vaporization is not clearly indicated in any other sample.
In the 2000C treated sample, Figure 4.11(b), a glass transition is seen at about
2000C, as indicated by the step increase in heat capacity. A weak response at about 3000C
is also seen. These data are in good agreement with the DMA results shown in Figure
4(b).
In both the 3000C air-cured and nitrogen-cured samples, glass transitions are seen at
3000C. The transition in the air-treated sample, Figure 4.11(c), is broader and slightly
shifted to a higher temperature, as compared to the transition in the nitrogen-treated film,
Figure 4.11(d). This is consistent with the DMA scans shown in Figure 4.4(c,d).
However, in the DSC scans, a weak step in the heat capacity is seen at about 3300C, where
no comparable change in mechanical modulus was observed in the DMA data.
168
o
0
0
0
E
0O
2,
(sl!un AJeJl!qje) eoueqJosqv eA!,eleH
Figure 4.6. Relative absorbance vs. wavenumber (400-2000 cm - 1) for spin coated
Probimide® 32 films. Arrow points to 1680 cm - 1 carbonyl peak: a.) 10 minutes at 100 0C,
169
o
0
0O
E
cc
LO
0
0l
OC
o
o
(sp!un Ajejl!qJe) eoueqjosqV eA!lelae
Figure 4.6. continued. b.) then 10 minutes at 120 0C,
170
LO
o
E
0O
ow
eoueqJosqVeAlec
(sun ) AJl!qJ
(sl!un AWeJ4!qw) eoueq~osq~
Figure 4.6. continued. c.) then 10 minutes at 140 0 C,
ea!4eje~
--
171
0
0
0
0
E
0.
I,,
E
,,
0U)
0n
0
Cun
ueqosq
el
(sp~un Aawl!qje) eoueqaosqV8Alj~ejO
Figure 4.6. continued. d.) then 10 minutes at 160 0C,
172
o
0
LO
0
0
E
.0
0)
(Q
(sl!un Ajejlqae ) 0oueqaosq-V O~llelati
Figure 4.6. continued. e.) then 10 minutes at 180 0 C,
173
o
0
LO
0
E
E
LO
OCN
(sl!un AjeJl!qJe) eoueqJosqV eA!IeBll
Figure 4.6. continued. f.) then 10 minutes at 2000 C.
174
0
i
a
E
L.
MW
~
0
0
(sl!un •Je~j!qJe) aoueqJosqy VA!lelGJ
Figure 4.7. Relative absorbance vs. wavenumber (2000-4000 cm-1) for spin coated
Probimide® 32 films: a.) baked 10 minutes at 100 0C, b.) then 10 minutes at 120 0 C, c.)
then 10 minutes at 140 0 C, d.) then 10 minutes at 160 0C, e.) then 10 minutes at 180 0 C, f.)
then 10 minutes at 200 0C.
175
Lu
C)
.0
E
C)
(sl!un Ajej!qje);eoueqJosqv GAPIele1
Figure 4.8. Relative absorbance vs. wavenumber (400-2000 cm - 1) for spin coated
Probimide® 32 films: a.) 15 minutes at 100 0 C (arrow points to 1680 cm -I carbonyl peak),
176
o
0
LO
E
0)
CD
(sl!un AjeaJ!qJe) eoueqjosqVoq
eA!•elt
E
Figure 4.8. continued. b.) then 1 hour at 200 0 C (arrow points to 1680 cm - 1 carbonyl peak),
177
I-
--------
~~I
---
--
~---------
-D0LO
c
· ~O
I
0
-0 0
E
0
nn
E
LO
C)
t-o
j
I- 1-----~--(sl!un AJeJ.!qJe) eoueqjosqVi
--- I
-D
ea!Iejea
Figure 4.8. continued. c.) then 1 hour at 300 0 C in either air or (arrow points to 1510 cm-1
C=C peak),
178
o
0
U)
0
E_.
t*
(sl!un Ajejl!qje) eoueqJosqV eanlCelEi
Figure 4.8. continued. d.) nitrogen (arrow points to 1510 cm - 1 C=C peak),
179
0D
0
cD
0
0
0
LnE
0.._
LM.
0
oE
M
LO
(sp!un Ajejl!qJe) aoueqjosqV a^JelaOE
Figure 4.9. Relative absorbance vs. wavenumber (2000-4000 cm -1) for spin coated
Probimide® 32 films: a.) baked 15 minutes at 100 0 C, b.) then 1 hour at 2000 C, c.) then 1
hour at 300 0C in either air or, d.) nitrogen.
180
1.5
1.0
0.5
II
0.0
II
Highest Cure Temperature (oC)
Figure 4.10. Ratio of relative absorbance of peak at 1680 cm - 1 to that of the 1370 cm - 1 vs.
highest cure temperature.
181
181
0
Lt
Cn
0
0
CD)
0
Ln
C14
0
CD
CJ
C*4LC
4)
E
C)
0
Lt
0
0
0
In
qf
0
<
-
I
0
0
0
(I/M) MOI- leaH o!WJuLaqIo0PU3
Figure 4.11. Endothermic heat flow vs. temperature for Probimide® 32 thick films: a.)
baked 6 hours at 100 0C,
182
o
0
0
0
0
CO
CD
CDO
U)
0
0
CD
oQ)c
CL
E
CDC
CD
=I-
0
0V
0)
3-
o(51M) MJ
1
MOl_-I •,•eH
o
=
Ig•
oI
0
0
LOn
O!LWeqlopu3
(DIM) MOld IeaH o!WueLHyopu3
Figure 4.11. continued. b.) then 3 hours at 2000 C,
183
o
0
LOl
cv,
0
0t•M
LO
oT
o
C C
CD
4)
-ca
1-0
0
0
u,
C
I 1
d
(D1M) MOlI- leaH 3!luJeqLopu3
Figure 4.11. continued. c.) then 3 hours at 300 0C in either air or,
0
0
·I
184
o
LO
0o
cy,
0
Cn
0
CD
0
LN
0
1)
(N o
L.
E
i-r)
C
r
-
) Mq
Ci
(5/AA) Mo13 lean o!ruJlelopu3
Figure 4.11. continued. d.) nitrogen.
o
185
4.4. Conclusions
Although the polyamide-imide Probimide ® 32 undergoes no thermal imidization
reaction during curing, the structure and properties of the polymer system are dependent
upon its curing conditions. The effects of progressive cure conditions on the properties
and physiochemistry of the polymer have been studied by several characterization methods.
When bulk films of Probimide® 32 are treated at only 1000 C, a great deal of solvent
remains. The residual solvent affects the properties and deteriorates the thermal stability of
the material. The volume of residual solvent is reduced and the properties and thermal
stability improved when the polymer is cured to 2000 C. Samples treated at 300 0 C were
solvent free and were highly thermally stable. However, substantial oxidative crosslinking
occurs if the polymer is cured at 3000 C in air rather than in an inert environment such as
nitrogen.
4.5. References
1. C.E. Sroog, MacromolecularReviews, Vol. II, J. Polym. Sci. (1976).
2.
S.D. Senturia, in Polymersfor High Technology - Electronicsand Photonics,
M.J. Bowden and S.R. Turner, Eds., American Chemical Society, Washington,
D.C., 1987, p. 428.
3. J.W. Verbicky, Jr., "Polyimides" in Encylopedia of Polymer Science and
Engineering, J.I. Kroschwitz, Ed., John Wiley and Sons, Inc., New York, 1987.
4.
H.R. Slater, Advanced Materials and Processes, 1, 19 (1995).
5.
R.J. Karcha and R.S. Porter, Polymer, 33(22), 4866 (1992).
6.
S. Palsule and J.M.G. Cowie, Polym. Bull., 33(2), 241 (1994).
7.
D.C. Rich, E.K. Sichel, and P. Cebe, MRS Fall Meeting 1995, in press.
8. M.H. Kailani, C.S.P. Sung, and S.J. Huang, Macromol., 25, 3751 (1992).
186
9.
P.R. Dickinson and C.S.P. Sung, Macromol., 25, 3758 (1992).
10. E. Pyun, R.J. Mathisen, and C.S.P. Sung, Macromol., 22, 1174 (1989).
11. J.R. Ojeda, J. Mobley, and D.C. Martin, J. Polym. Sci. B, 32, 559 (1995).
12. J. Jou and P. Huang, Macromol., 24, 3796 (1991).
13. C.A. Pryde, SPE ANTEC '90, 439 (1990).
14. M.-J. Brekner and C. Feger, J. Polym. Sci. A, 25, 2005 (1987).
15. E.D. Wachsman and C.W. Frank, Polymer, 29, 1191 (1988).
16. R.W. Snyder, B. Thomson, B. Bartges, D. Czerniawski, and P.C. Painter,
Macromol., 22, 4166 (1989).
17. OCG Microelectronic Materials, Inc., Probimide 32 product bulletin.
18. E.g., C.J. Pouchert, The Aldrich Library of Infrared Spectra Edition III, Aldrich
Chemical Company, Inc., Milwaukee, WI (1981).
19. R. Silverstein and G. Bassler, Spectrometric Identification of Organic Compounds,
John Wiley & Sons, New York (1967).
20. S. Kuroda and I. Mita, Eur. Polym. J., 25(6), 611 (1989).
21. R. Dine-Hart, D. Parker, and W. Wright, Br. Polym. J., 3, 222 (1971).
22. R. Dine-Hart, D. Parker, and W. Wright, Br. Polym. J., 3, 226 (1971).
23. H. Ishida, S. Wellinghoff, E. Baer, and J. Koenig, Macromol., 13, 826 (1980).
24. L. Broadbelt, S. Dziennnik, and M. Klein, Polym. Degr. Stab., 44, 137 (1994).
187
Chapter 5. Alignment of Liquid Crystals on Brushed
Polymer Layers.
5.1. Introduction
Despite the tremendous industrial importance of, and scientific interest in, the
surface alignment of liquid crystals on polymer films, it is still a poorly understood
phenomenon. Berreman and others [1-3] have argued that the liquid crystals align along
microgrooves created by the rubbing. The irregularity in size and location of these
scratches [4-8] in buffed polymer films, however, might cast some doubt upon this theory.
On the other hand, researchers have shown that the non-rub formation of microgroove
patterns can in fact produce alignment of nematic liquid crystals [9,10]. For example,
Kawata et al. [10] have shown that liquid crystal alignment can be achieved using fine
microgroove surface patterns with pitches less than 2gm produced by photolithography.
More recently, authors have suggested that molecular orientation of the main chains
produced by the brushing is responsible for alignment [8,11-19]. For example, Aoyama et
al. [11] have shown that liquid crystals can be aligned on polymer films that have been
stretched but not scratched. In addition, authors have observed that the direction of
alignment is not always identical to the direction of the scratches. For example, while the
grooves in rubbed polystyrene films run parallel to the brushing, the liquid crystal
alignment is perpendicular to the brush direction [6-8]. This suggests that orientation of the
phenyl side groups, which are approximately perpendicular to the main chain, might be
responsible for alignment in these films. It seems reasonable that if the physical groove
pattern were the only factor responsible for alignment, then nearly any grooved polymer
film should be able to align liquid crystals and the alignment should always occur in the
same direction.
Geary et al. [13] have suggested that high crystallinity in polymer films is
associated with the polymer's ability to align liquid crystals. Russell [18] reports, using a
surface-sensitive X-ray reflectivity technique, that brushed polyimide films develop an
oriented crystalline structure at the surface. On the other hand, polyimides, though widely
used for liquid crystal alignment, are not widely known for being crystalline. The BPDAPDA polyimide studied by Russell [19] is one that is known to develop high degrees of
crystallinity [20], which is an unusual characteristic for polyimides. Though polyimides
are typically amorphous, it is possible that crystalline structures could form at the polymer
188
surfaces. Factor et al. [21], for example, report crystalline structure on the top 90A of
PMDA-ODA polyimide films.
Polyimides with long alkyl side groups and bulky fluorinated groups are frequently
used for liquid crystal alignment. These alignment materials are known to generate large
pretilt bias angles in the liquid crystal layer [22-27]. In addition photocrosslinkable
polyimides are commercially available for liquid crystal alignment applications [28]. These
polymers allow the alignment films to be patterned by photolithography. It is unlikely that
any of these polyimides could crystallize due to the bulky side groups or to the
photocrosslinked main chains. Hence, this casts substantial doubt that crystallinity could
be a necessary feature for liquid crystal alignment.
In conclusion, the surface alignment of liquid crystals on polymer layers is not well
understood despite the significant amount of work that has been done. Conventional thin
film and surface evaluation techniques do not seem to be generating highly insightful
information regarding the phenomenon. In this chapter, the alignment of liquid crystals on
Probimide 32 polyamide-imide and other polymer layers is demonstrated and discussed. A
optical transmission technique for quantitatively evaluating parallel alignment is presented.
A highly novel technique for investigating thermophysical transitions in surface layers is
then demonstrated.
5.2. Experimental Section
The following section describes the experimental techniques used for producing and
evaluating liquid crystal alignment cells. It was necessary to use laboratory techniques that
optimally reflected the goals of, and were realistic to, the resources available. Thus,
extensive consultations with LCD researchers at the 3M Corporation [29] were helpful in
developing the experimental approach described in this section. The goals of these
experiments were to:
1. Obtain data that are physical meaningful and have real scientific merit.
2. Present proof of new concept research on the laboratory scale.
3. Focus on the materials science of the liquid crystal surface alignment phenomenon,
rather than on LCD device technology in general.
189
Many decisions were made during the course of this project in order to serve and
focus upon the above goals. For example, it was decided to build cells in a university
laboratory rather than in an industrial clean room. A hand brushing technique was used
rather than a commercial buffing wheel for the induction of alignment. Merck E7 liquid
crystal mixture, which is mainly used in research rather than in commercial displays, was
selected as the nematic material. Parallel nematic alignment was chosen as the geometry for
the evaluation of alignment. The techniques described below are optimal methods with
respect to the purposes of this research.
5.2.1. Construction of liquid crystal alignment cells
Preparation of substrates
The following procedure was adopted for preparing glass substrates for spin
coating. The purpose of the following cleaning procedure was to remove grease, dirt, and
dust particles from the glass. This was desired so that the substrate coverage and
smoothness of the polymer coating could be optimized. Poor substrate preparation resulted
in poor polymer coatings.
1. Glass microscope slides (25 x 75 x 1 mm) were cut approximately in half by
mechanically etching the slide down the middle along its width. The etched slides were
then gently broken in half.
2. The slides were blown clean of particles and dust with an air jet stream.
3. The slides were soaked for 15-20 minutes in isopropyl rubbing alcohol for removal of
organic contaminates.
4. The slides were rinsed of the alcohol by soaking for 15-20 minutes in distilled or
deionized water.
5. The slides were cleaned in an ultrasonic bath with detergent for 20 minutes.
6. The slides were rinsed in distilled or deionized water for 15-20 minutes.
190
7. The slides were rinsed again in an ultrasonic bath of distilled or deionized water. It was
found that this dual rinse procedure was necessary for thorough removal of the
detergent. Residual detergent was found to have a significant adverse affect on coating
quality.
8. The wet slides were blown dry with an air jet stream.
9. The slides were stored in a PTFE, low-dust sample holder which held twenty glass
slides.
Spin coating procedure
The following general procedure was adopted for applying polymer alignment films
to the cleaned glass substrates. I specifically thank Prof. Anne Mayes for use of the
Headway Research spin coater.
1. A dilute solution of the polymer was prepared. The primary polymer of interest,
Probimide 32 polyamide-imide, was obtained from OCG Microelectronic Materials,
Inc., as 21 wt% solids and was diluted to approximately 3 wt% solids with N-methyl
pyrrolidone (NMP). For example, to 2.5 mL of Probimide 32 original solution, 15 mL
NMP were added.
2. The glass substrates were blown clean of dust with an air jet just prior to spin coating.
3. The glass substrates were prewet with solvent, which was spin coated at 4000 RPM for
30 seconds. For Probimide 32 coatings, the substrates were prewet with NMP.
4. The polymer solution was spin coated onto the prewet glass substrates at 4000 RPM
for 30 seconds.
5. The films were softbaked about 15-20 minutes at approximately 110 0C at atmospheric
pressure on a hot plate or on a temperature-controlled metal block in a National
Appliance Oven. The films appeared dry after the softbake. The resulting Probimide
32 polyamide-imide films were approximately 450A thick, as measured by a Dektak
8000 profilometer.
191
Alignment layer buffing procedure
The following technique was used to brush the polymer films for liquid crystal
alignment. The procedure is depicted in Figure 5.1. The substrates were temporarily
adhered with two-sided tape in a locked position to a glass plate, which was in turn,
securely taped to a horizontal bench top. A rayon cloth was wrapped around a cylindrical
bar which weighed approximately 1 lb. The bar was dragged across the polymer film by
hand exactly 25 times such that the vertical force on the film was equal to that of the weight
of the bar. Although this technique is not as controllable as a commercial brushing wheel,
researchers at 3M Corporation believe this technique to be highly adequate in producing
reasonably consistent alignment films [28].
Buffing cloth
Alignment layer surface
Figure 5.1. Alignment film brushing technique.
Liquid crystal alignment cell building procedure
Liquid crystal cells were constructed by the following procedure, which is depicted
in Figure 5.2.
1. Two alignment films on glass were prepared and brushed as described above.
2. PF-70 7gpm diameter microrod spacers obtained from Nippon Electric Glass were
mixed into a globule of Norland Optical Adhesive 61 UV-curable epoxy (Norland
192
Products, Inc., New Brunswick, NJ 08902). The mixture was mixed fresh prior to
cell building. A tiny clump of the spacer rods, approximately 0.03 mg, was added to
around 3 drops of the epoxy. This was enough mixture to make about 4 cells.
3. The epoxy/microrod mixture was placed near the edges of one substrate parallel to the
brush direction.
4. The two slides were pressed firmly together with the alignment films on the inside of
the cell and the brushing directions parallel.
5. The epoxy was exposed through the top glass substrate to i-line (? = 365 nm)
ultraviolet light under a Spectroline BIB-150B black light UV lamp (average area
normalized energy delivered to glass approximately 2.5 mW/cm 2 ) for about 3.5
minutes. The cell was about 10" from the bulb. The energy required to cure the UV
sensitive epoxy was many orders of magnitude less than what might affect the polymer
films.
6. The cell gaps were filled by capillary action with Merck E7 liquid crystals. The E7
liquid crystals are low-cost monomeric nematic liquid crystals used primarily in
laboratory research. The chemical composition of the liquid crystals is proprietary,
though Cognard [30] reports from chromatographic analysis the materials to be the
mixture of molecules shown in Table 5.1. The physical properties [31] of the E7 liquid
crystals are listed in Table 5.2. The liquid crystals were filled parallel to the direction of
alignment, a technique which appeared to minimize the cell defects.
7. The cells were maintained at about 800 C on a temperature-controlled metal block in a
National Appliance oven for approximately one-half hour and then cooled. This
treatment was sufficient to clear the liquid crystals and minimize defects.
8. The cells were sealed with Devcon 5 Minute epoxy so that the liquid crystals could not
seep out of the cells.
193
(a) COAT TWO ALIGNMENT
FILMS ONTO GLASS;
APPLY BRUSH TECHNIQUE
(b) CONSTRUCT
CELL WITH GAP
711 SPACER RODS
+ EPOXY
(c) FILL GAP WITH NEMATIC
LIQUID CRYSTALS
SPACERS & EPOXY
CI
Z' i
I
ADHERED GLASS SLIDES
-.
m
MERCK E7 NEMATIC
LIQUID CRYSTALS
(d)MAINTAIN FILLED CELL
ABOVE THE LC CLEARING POINT
I
--
I
T
. .w
(e) SEAL COMPLETED CELL
Figure 5.2. Cell construction procedure. (a) Alignment film formation, (b) Empty cell
construction, (c) Cell filling, (d) Cell annealing, and (e) Cell sealing.
194
Table 5.1.
Composition of Merck E7 nematic liquid crystals from chromatographic
analysis [30].
I
_ _
51%
_~
__
1~1
~_
_ I_
_
NC
~
//
~C
25%
8%
I~ __ _
NC--
NC
_I
1(~1_
_
_~
~C
I~
16%
_
C5Hj
1
~
NC
I_
__~_
I_ ~__ __
7
_
H15
~ICI
_
-Cs 8H,7
C5H1
_
195
Table 5.2. Physical properties of Merck E7 nematic liquid crystals [31].
Melting Point
Liquid Crystal Clearing Point
Extraordinary Refractive Index at 589 nm
Ordinary Refractive Index at 589 nm
Birefringence
Viscosity (200C)
-100C
60.50C
1.746
1.522
0.224
39 mm 2/s
5.2.2. Evaluation of alignment cells
Oualitative evaluation of alignment
The liquid crystal alignment in the cells was judged qualitatively by illuminating and
rotating the cells between two crossed polarizers. The observations made for aligned and
unaligned cells are summarized in Figure 5.3. This technique is an excellent first order test
of an alignment film's ability to orient the liquid crystals macroscopically.
Aligned cells exhibited macroscopic optical birefringence, with four extinctions
occurring 900 apart upon rotation when the alignment direction is parallel to either polarizer
axis (Figure 5.3(a)). Light is transmitted when the cell is rotated off one of these
orientations. The maximum transmitted light intensity occurs when the nematic director is
450 to the polarizers' axes (Figure 5.3(b)).
In contrast, unaligned liquid crystal cells were bright (i.e., they evidenced no
extinction of light) for all rotations of the cell between crossed polars (Figure 5.3(c)). This
is because the unaligned liquid crystals form arbitrary nematic phases with randomly
varying nematic directors. The birefringence is microscopic but not macroscopic.
196
A
(a)
LP
ALIGNMENT
A
(b)
tP
(c)
A
I
L,
NO ALIGNMENT
Figure 5.3. Qualitative observations of liquid crystal alignment cells between crossed
polarizers. For aligned cells, a.) the light is extinguished when the alignment director is
parallel to either polarizer, and b.) maximum intensity is transmitted when the alignment
director is 450 with respect to the polarizers. For unaligned cells, c.) the cell appears bright
for all rotations.
197
Measurement of optical transmission
The optical transmission of the liquid crystal cells between a crossed polarizer (P)
and analyzer (A) was measured using the configuration shown in Figure 5.4. The laser is a
Spectra-Physics Model 159 He-Ne laser (, = 6328A). Phase sensitive detection was
achieved by use of a Stanford Research Systems SR540 chopper and EG&G Model 5209
lock-in amplifier. A diagram of the connections is shown in Figure 5.5, and the settings
for the lock-in amplifier are listed in Table 5.3. A Newport 818-SL detector with 883-SL
attenuator and Model 835 power meter were used to detect transmitted intensity. The time
constant for intensity measurements was 1 second. A chopping speed of 100 Hz was
used. The polarizers had an extinction ratio (intensity A//P / intensity AiP) of about 105.
The intensity transmitted by the crossed polarizers was negligible compared to the lowest
cell transmission measurement.
Measured voltage was linear with light intensity for the range studied. This is
proven in Figures 5.6 and 5.7. First, a series of calibrated neutral density filters were
placed in front of the detector with no other components in the optical train. The measured
relative intensity is plotted in Figure 5.6 against expected relative intensity, which was
calculated according the extent of calibrated neutral density filtration. Figure 5.6 indicates a
linear relationship between real and measured intensity. The data is plotted on a linear scale
in Figure 5.6(a) and on a log-log scale in Figure 5.6(b) so that both data approaching one
and data approaching zero can be seen.
Next, a polarizer and analyzer were placed in the beam. The transmitted intensity
was measured as the analyzer was rotated with respect to polarizer. Expected relative
intensity was calculated according to the theoretical relationship:
I/E2 = cos 2 X
(1)
where I/E 2 is relative intensity and X is the angle between the polarizer and analyzer.
Measured relative intensity is plotted against expected relative intensity in Figure 5.7. The
data are plotted on a linear scale in Figure 5.7(a) and on a log-log scale in Figure 5.7(b) so
that both data approaching one and data approaching zero can be seen.
198
SAMPLE
CHOPPER
P
A
LASER -- -
RI
Figure 5.4. Optical transmission apparatus with phase sensitive detection.
199
LOCK-IN AMPLIFIER
[REFERENCE AC IN]
[ANALOG OUTPUT]
-
OPTIC AL
POWE R
METE R
[REFERENCE
M ODE
f]
CHOPPER
CONTROLLER
III
[CHOPPER
HEAD CABLE]
[DETECTOR]
DETECTOR
HEAD
0
CHOPPER
HEAD
Figure 5.5. Instrument connections for phase sensitive detection. Port labels are stated in
brackets where appropriate.
200
Table 5.3. Settings for the EG&G 5909 lock-in amplifier used for phase senstivitve
detection of chopped light.
SENSITIVITY
Input from voltage source (i.e. power meter) at A port and press A key
Adjust sensitivity scale with up and down keys as appropriate
FILTERS
Use Bandpass BP filter mode.
Use TRACK to track the reference chopping frequency.
Set LINE REJECT such that neither F nor 2F filters are lit.
TUNING
Hit red AUTO key, then AUTO PHASE key, and wait for red AUTO light to turn off.
This procedure automatically determines the phase adjustment for maximum output signal.
The adjustment is necessary because the external electronics are not necessarily in phase.
Use +90' to shift the phase adjustment by 900, twice to shift by 1800
REFERENCE
Connect chopper signal at AC IN port
INT (internal reference) should not be lit, since an external reference is being used
2F should not be lit, as this would double the reference frequency
OUTPUT
Adjust TIME CONSTANT as desired
The higher the time constant, the less the noise, but the longer it take to produce data
10 sec is a good time constant to begin with
Use SLOPE 6dB, though this is not crucial in most cases
DYN RES - NORM
DISPLAY - %FS.SIGNAL (% of full scale sensitivity setting of output signal)
OFFSET & EXPAND - keep unlit
201
(a)
*
0.8-
S
S
0.60
a:
0
0.40
0.20
p
0.0
I
.
0.0
*
*
0.2
0.4
0.6
0.8
Expected Relative Intensity
1.0
(b)
0
m
.01
"
.001
*
I
.0001
.0001
0
.001
Expected
.01
.1
1
Relative Intensity
Figure 5.6. Measured relative intensity vs. expected relative intensity for phase-sensitive
detection of neutral density filtered light. Expected relative intensity is calculated from the
extent of calibrated neutral density filtration. a.) linear scale, and b.) log-log scale.
202
1.0
h
-
0.8
-
ftt
0.6
0.4
0.2
a~
-
00 < X < 900
900< X< 1800
-
a
-
on
m
--
L VI1
v.v
0.0
I
a
I
I
0.2
0.4
0.6
0.8
1.0
Expected Relative Intensity
(b)
I
.1
a
00 < X < 900
.01 1
900< x < 1800
ft
.001 -
.0001
2i
-
.0001
. . .....
.001
.01
.1
1
Expected Relative Intensity
Figure 5.7. Measured relative intensity vs. expected relative intensity for phase-sensitive
detection of light diminished by two consecutive polarizers with their polarizing axes lying
at an angle X. Expected relative intensity is given by cos 2X. a.) linear scale, and b.) loglog scale.
203
The optical transmission intensities of the liquid crystal alignment cells were
measured between the crossed polarizer and analyzer. To calculate the alignment parameter
A=I(0 0)/I(45 0) discussed in Chapter 1, both the intensities I(00), brushing axis parallel to
the polarizer, and 1(450), brushing axis 450 to the polarizer were measured. Approximately
12 measurements were taken for each orientation, i.e., 00 or 450, of each sample cell. The
measurements were taken for sample spots distributed throughout the cell. Highest and
lowest values of the data sets were discarded and average values were calculated from the
remaining measurements. The experimental error is discussed in Section 5.4.
The optical transmission of the liquid crystal cells was measured while adjusting for
very small (up to 30) Maguin twist distortions [32] in the cells that appear to have been
introduced during cell construction. Such distortions rotate the axis of polarization of the
light, but do not represent defects in the surface alignment of the liquid crystals. The
analyzer and cells were adjusted such that for the 00 state, the polarizer was parallel to the
first LC surface director and the analyzer was perpendicular to the second LC surface
director. This is shown in Figure 5.8. The adjustment should counteract the affect of the
twist distortion. Failing to adjusting for twist distortions in the 00 extinction position can
have a serious affect on the accuracy of the data. The small twist distortions should not
have a significant affect on the intensity at the 450 orientation, as can be shown by equation
(25) of Chapter 1, and it does not matter therefore whether an adjustment is made for this
position.
Twist distortions can also introduce ellipticity in the light passing through even if
the components are adjusted as shown in Figure 5.8. However, one can show that for
small twist angles <30, by making this adjustment, the ellipticity of the transmitted light due
to twisting is very small, and that cell can be reasonably approximated as a parallel-aligned
layer. According to Gooch and Tarry [33], after making the adjustment shown in Figure
5.8, the relative transmission (T) due to elliptically polarized light is given by:
T = [sin 2 (0 (1+u2) 1/2 ] / [1+u 2 ]
(2)
where 0 is the twist angle and u is given by:
u = tihAn/6k
(3)
where h is the liquid crystal layer thickness, An is the birefringence, and X is the
204
wavelength of light in free space. If we assume h=7.m, An-0.224, X=6328A, and for the
worst case scenario, 0=30, the calculated value for T is approximately 4.5x10 -5 , which is
more than two orders of magnitude below the lowest measured value of cell transmission.
Hence, it is reasonable to conclude that ellipticity due to twist distortion is not significant.
P.N
Figure 5.8. Adjustment made for a cell with small degree of twist distortion 0. The first
liquid crystal surface director N is oriented parallel to polarizer P. The second surface
director is N'. The analyzer is adjusted by 0 from A to A' such that A' is perpendicular to
N'.
205
5.3. Results and Discussion
5.3.1. Brushing induced alignment in Probimide 32 polyamide-imide
The processing, structure, and properties of Probimide 32 polyamide-imide were
previously discussed in Chapter 4. In this section, it is shown that this polymer can be
used as an alignment layer for nematic liquid crystal. Alignment films of Probimide 32 and
liquid crystal parallel alignment cells were produced by the techniques described in Section
5.2. The brushing method was used to induce the liquid crystal aligning properties in the
polyamide-imide. Qualitatively, it was apparent that the liquid crystal cells produced by
this method were strongly aligned along the brushing axis.
The alignment of the liquid crystal cells was evaluated by use of the laser optics
method described in Section 5.2. A brush-aligned cell was placed between the crossed
polarizer and analyzer. The azimuthal angle of the alignment director with respect to the
polarizer was varied. The transmitted intensity is plotted in Figure 5.9(a) as a function of
the azimuthal angle of the alignment director. The intensity axis is normalized by the
transmitted intensity measured with the alignment director at 450 with respect to the'
polarizer. For ideal alignment, we expect the transmitted intensity to follow the expression:
I(4)/I(450) = sin 2 (2))
(4)
where 0 is the angle of the alignment axis with respect to the polarizer and I(4)/I(450) is the
transmitted intensity normalized by that at 0=450. The theoretical relationship is plotted in
Figure 1.13 in Chapter 1. It is apparent that the measured relationship shown in Figure
5.9(a) closely approximates the theoretical relationship in Figure 1.13. Thus, it is clear that
there is a strong degree of alignment in the brush aligned cell. On the other hand, the
intensity at 0', 900, and 1800 is not zero as theoretically predicted, indicating that the
alignment of the liquid crystals is not perfect. The ratio of minimum to maximum intensity
is on the order of 10-3.
The optical transmission of a liquid crystal control cell in which there was no
brushing treatment to the alignment film is shown in Figure 5.9(b). Qualitatively, no
alignment was observed in this cell. The cell was placed between the crossed polarizer and
analyzer of the laser optical transmission apparatus. The length axis of the cell (which
would have been the brush axis had the alignment films been brushed) was selected as the
axis of reference. The cell was rotated through 1800 and relative intensity was plotted
206
(a)
1
.01
.Ol
•
.001
I
.0001
45
0
135
90
Angle (degrees)
180
(b)
C
I~-
---
~--~--~
01
.001
0.001
.0001
-
0
45
90
135
Angle (degrees)
I
180
Figure 5.9. a.) Intensity / intensity at 450 vs. rotation angle for a nematic liquid crystal cell
aligned with brushed Probimide 32 polyamide-imide layers. The x-axis is the azimuth of
the alignment director with respect to the polarizer. b.) Relative intensity vs. rotation angle
for nematic liquid crystal cell with untreated Probimide 32 polyamide-imide layers. The xaxis represents the azimuth of an arbitrary reference axis with respect to the polarizer.
207
against the angle between the polarizer axis and the reference axis. It is apparent that,
unlike the aligned cell, there is no systematic change in the transmitted intensity as the
sample is rotated. This indicates that there is no macroscopic alignment in this sample.
For all aligned liquid crystal cells produced with brushed polyamide-imide layers,
the average ratio of intensity at 00 to intensity at 450 is approximately 1.04 x 10-3 with a
standard deviation of 2.8 x 10-4 (27%). This ratio, which is referred to as the A parameter
in Chapter 1, is three orders of magnitude lower than unity, the value of the A parameter
which should occur in the case of no macroscopic alignment. Hence, it is clear that there is
very strong and significant alignment in these specimens. The effective angle of
misalignment (D,defined by:
A - sin 2 (2i)
(5)
is approximately 0.920 for the brushed polyamide-imide aligned cells. Since 4=00 for an
ideally aligned specimen and •=45 0 for an unaligned specimen, it is quite apparent that the
misalignment of the liquid crystals with respect to the alignment director is very small.
5.3.2. Brushing-induced alignment in other polymers
During the course of these investigations, it was desired to find other polymers
which effectively align the E7 nematic liquid crystals by the rubbing technique. A brief
discussion of the results of this search is worthwhile. In Table 5.4, polymers which were
used as alignment films are listed. Their success in parallel-aligning the liquid crystals in
the brush direction is stated in the second column of Table 5.4. Some characteristics of the
polymeric materials are listed in the remaining columns. The polymers in Table 5.4 were
said to have a "rigid backbone" if the repeat unit contained ring structures or bulky side
groups. The polymers were said to have "strong intermolecular forces" if the repeat unit
contained highly dipolar moieties, such as those involved in hydrogen bonding or charge
transfer complexation. Calculated effective angles of misalignment for cells produced with
various polymer layers are shown in Figure 5.10.
Two characteristics which appeared not to be necessary for aligning the E7 liquid
crystals were backbone rigidity and aromatic content. Since aromatic polyimides have both
these characteristics, one might have speculated that these characteristics may be necessary
for, or associated with, liquid crystal aligning properties. However, this does not appear to
be the case. For example, poly(vinyl alcohol), a non-aromatic vinyl polymer, was an
effective alignment layer material, while polycarbonate, a rigid aromatic polymer, was not.
208
The polymers with strong intermolecular forces, on the other hand, appear to be
strong aligners of the E7 liquid crystals. Polyimides are, for example, known to exhibit
strong intermolecular charge transfer interactions. The polyamic acid, though weaker in
charge transfer, contains both carboxylic acid and amide groups, which are strongly
hydrogen bonding moieties. The polyamide-imide is a hybrid of the polyimide and the
polyamide and should also exhibit strong intermolecular forces. Poly(vinyl alcohol)
contains strongly hydrogen bonding hydroxyl groups. The poly(phenylene ether sulfone)
molecule contains highly electron withdrawing sulfone groups, which should engender
strong permanent dipoles in the main chain. All of these polymers have a yellowish
appearance in the solid film state, which might suggest a helpful rule of thumb in selecting
a good alignment polymer.
Table 5.4. List of polymers which were examined as liquid crystal aligning agents and
some of their characteristics.
Polymer
Probimide 32
polyamide-imide
Aligns
Rigid
Aromatic
Strong Inter-
E7 LCs?
Backbone?
Groups?
molecular Forces?
yes
yes
yes
yes
Color
yellow
(PAI)
PMDA-APB
polyamic acid
yes
yes
yes
yes
yellow
(PAA)
PMDA-APB
yes
yes
yes
polyimide (PI)
poly(vinyl
yes
.
yes
no
no
yellow
yes
alcohol) (PVA)
poly(methyl
methacrylate)
yellow
no
no
no
no
none
(PMMA)
polycarbonate
no
yes
yes
no
yes
yes
yes
yes
none
(PC),
poly(phenylene
ether sulfone)
(PPES)
yellow
209
7
4035
rn
-
30-
VU
25201510-
i
50*
07ý4
M!2=.
cnsmn
PI
PAA
PAl
PPES
""7""
PVA
PMMA
PC
Polymer
Figure 5.10. Values of D, the effective angle of misalignment, for liquid crystal cells
produced with alignment films of brushed PMDA-APB polyimide (PI), PMDA-APB
poly(amic acid) (PAA), polyamide-imide (PAI), poly(phenylene ether sulfone) (PPES),
poly(vinyl alcohol) (PVA), poly(methyl methacrylate) (PMMA), and polycarbonate (PC).
All cells but those produced with PMMA and PC appear to have very strong macroscopic
alignment.
5.3.3. FTIR Spectra of Probimide 32 polyamide-imide alignment films
FTIR spectra of Probimide 32 films are displayed and discussed in Chapter 4. The
films in Chapter 4, however, were spin coated from 2lwt% solids and were approximately
4.m thick. The polyamide-imide films used in this chapter for liquid crystal alignment
cells, on the other hand, were coated from 3wt% solids and are approximately two orders
of magnitude thinner.
The FTIR study in Chapter 4 showed that solvent was systematically removed from
the 4pm films as the material was heated from 100 0C to 2000 C. This was primarily
evidenced by the significant change in peak height of the 1680 cm - 1 amide carbonyl
vibration relative to the 1370 cm-1 imide N-C vibration. The 1680 cm-1 peak declined as
210
NMP solvent was driven from the film. The ratio of the 1680 cm-1 band to the 1370 cm- 1
settled near 0.6-0.7 at point of full solvent removal.
The solvent dynamics are significantly different, however, in the thin alignment
layer films. Figure 5.11 shows the FTIR spectrum of a thin Probimide 32 alignment layer
film on KBr which was cured by only the 110 0 C soft bake treatment. The 1680 cm- 1
vibration appears to be mitigated as compared to the solvent-containing films in Chapter 4.
The ratio of the peak height of the 1680 cm-1 band to that of the 1370 cm- 1 is about 0.63,
which is consistent with a solvent free film. Thus, this study indicates that the solvent in
the Probimide 32 preimidized polyamide-imide resin can be removed rapidly at low
temperatures in very thin films (approximately 450A).
Brushing the polyamide-imide film induces orientation of the polymer chains in the
direction of the brushing. This is consistent with what other researchers [8,11-19] have
found in brushed alignment films of other polymeric materials. The orientation is clearly
indicated by the dichroic FTIR spectra of brushed films in Figure 5.12(a,b). The
polyamide-imide films have been brushed, then scanned in the FTIR after placing a gold
wire grid IR polarizer in the path of the beam. For Figure 5.12(a), the IR is parallel to the
direction of brushing, and for Figure 5.12(b), the IR is perpendicular to the direction of
brushing. It is important here to notice the relative ratios of the 1370 cm- 1 imide N-C peak
to that of the 1720 cm- 1 imide C=O peak. These values are listed in Table 5.5. The 1370
cm- 1 band represents vibration parallel to the main chain, while the 1720 cm-1 represent
vibration approximately perpendicular to the main chain. The ratio of the heights of these
peaks increases approximately 37% from 0.63 to 0.86 as the brushed polymer film is
rotated from the perpendicular to the parallel position with respect to the IR polarization
axis.
5.3.4. Atomic Force Microscopy of brushed polamide-imide films
Atomic force microscopy (AFM) images are shown in Figure 5.13 for brushed and
unbrushed Probimide 32 polyamide-imide alignment layer films. For the brushed material,
Figure 5.13(a), there appear to be grooves running through the film parallel to the brush
direction. These grooves are not present in the unrubbed sample, Figure 5.13(b). These
relatively large scratches observed in the brushed material appear consistent with what other
researchers [4-8] have observed in brushed polymer layers. Seo et al. [5,7], however,
observed grooves structures in rubbed polystyrene and alkyl-branched polyimides, but not
in a brushed straight chain polyimides, which they observed to have only a rough, bumpy
surface.
211
I
0
-o
a
0
-oo
-
.
1L
-
0
.0
2
0)
-. 0
1o
0
(sp!un AjeiJ!qJe) eoueqJosqv a^!elal:
Figure 5.11. Relative absorbance vs. wavenumber for Probimide 32 polyamide-imide
coated from 3wt% solids content and softbaked at 110 0C, with final film thickness
approximately 450A.
212
0
0
0
o0
r
-
E
0
U
E
I
0
rC
)
0
0
0
0
0
(sl!un AjeJl!qJe) eoueqJosqv eAn!el'e
Figure 1.:z. Relative absorbance vs. wavenumber for a brushed Probimide 32 polyamideimide thin film. a.) Incident infrared electric field polarized parallel to the brushing
direction.
213
o
i
-o
o
-
I
o
0
-o
0
I
I
(sF!un AiJeJ!qje) eoueqjosqV ea!ielden
Figure 5.12. continued. b.) Incident infrared electric field polarized perpendicular to the
brushing direction.
214
zC
0
$~*::~I
C
I,
0
Ln
0Cý
U,>
0u
::
l
-0
of polyamide-imide films.
Figure 5.13. Atomic force microscopy (AFM) images
brushed.
215
0
II
fn
-0iLn
0
-N)
N
V
Figure 5.13. continued. b.) unbrushed
216
Table 5.5. Relative ratios of the heights of the imide N-C (1370 cm- 1) peaks to the imide
C=O (1720 cm- 1) peaks for brushed Probimide 32 layers.
Scan conditions
Absorbance Ratio
(1370 / 1720 cm- 1)
No brush, unpolarized IR
IR // Brushing
IR I Brushing
0.76
0.63
0.86
5.3.5. In situ thermal stability of liquid crystal alignment on brushed Probimide 32
polyamide-imide layers.
The surface alignment of the E7 liquid crystals on brushed Probimide 32 alignment
layers was stable to very high temperatures. A completely constructed cell was annealed
for half an hour at progressively higher temperatures in 20 0C intervals from 800 C to 180 0C.
The cell was cooled after each annealing and the alignment parameter A = 1(00)/1(450) was
measured. Treatment above 180 0C damaged the epoxy, thus ruining the cell. The quality
of alignment is plotted against annealing temperature in Figure 5.14. The data indicate that
the liquid crystals remain strongly aligned throughout the cure cycle.
-1
*
*0
.001
.0001
•
60
Il
.
80
"
.
100
"
.
120
*
"
.
140
0
"
I,
160
-I
-
180
200
Temperature (oC)
Figure 5.14. 1(00)/1(450) vs. temperature for a polyamide-imide aligned liquid crystal cell
undergoing an annealing cycle of systematically increasing temperature treatments.
217
5.3.6. "Liquid crystal alignment relaxation" - a new technique for measuring thermal
transition temperatures in surface layers.
The technique in the previous section for measuring the in situ thermal stability is
disadvantageous in that it is limited by the chemical thermal stability of the cell epoxy and
the liquid crystals themselves. The following technique was employed to measure the
thermal stability of only the polymer alignment films:
1. Coat and cure many polymer alignment layers.
2. Apply brushing treatment.
3. Cure the brushed alignment films at an array of temperatures (with at least two films per
treatment temperature). Hold each alignment film at a single temperature for 30
minutes, then cool.
4. Build liquid crystal cells from the post-brush cured alignment films.
5. Measure alignment strength A = I(00)/I(450).
6. Plot alignment strength 1(00)/1(450) vs. post-brush cure temperature.
Probimide 32 polyvamide-imide
For cells constructed with Probimide 32 polyamide-imide alignment films, the
alignment strength is plotted against post-brush annealing temperature in Figure 5.15.
There is a sharp break in the alignment near 2700C as 1(00)/1(450) changes from
approximately 10-3 to approximately 100. For post-brush temperatures 3000 C and above,
1(00)/1(450) remains around one, indicating that the alignment is completely destroyed.
The 300 0C "alignment relaxation temperature" correlates with the glass transition
temperature for solvent-free Probimide 32 found in Chapter 4. The change that occurs in
the polymer surface layer can be visualized as shown in Figure 5.16. The original coated
surface begins as an isotropic random coil in the plane of the film. The brushing process
orients the polymer chains in the direction of brushing. This orientation process is
associated with the layer's ability to align liquid crystals in the brushing direction. When
the material is treated above its glass transition temperature, however, the oriented structure
218
relaxes and the surface's ability to align liquid crystals is irreversibly lost. The alignment
with this polymer layer, thus, appears to be associated with the frozen-in ordering of
amorphouschains which relax at Tg.
The relaxation of the aligning ability of the polymer layer suggests a new technique
for measuring the thermophysical transitions in polymer surfaces. It also suggests a way in
which processing / structure / property studies of liquid crystal alignment surface layers can
be performed. For example, it was observed in Chapter 4 that residual solvent in
Probimide 32 bulk films substantially reduced the mechanical softening temperature. The
single glass transition temperature observed for the polyamide-imide alignment films near
300 0 C suggests, in agreement with FTIR results in Section 5.3.3, that the 110 0C treated
alignment films are essentially free of solvent.
This technique is not a direct measure of the bulk Tg such as that obtained by DSC
or DMA. This technique measures the ability of the surface to perform a function, i.e.,
align nematic liquid crystals. Hence, the relaxation technique measures only the
thermophysical properties of the material responsible for aligning the liquid crystals.
Authors [18] have suggested that only the first 10-60 nm is responsible for the alignment.
It is therefore unclear at this point whether this functional Tg is that of the surface
monolayer or of the first several nanometers.
1U
.**
tn
o
OO
.1 1
.01
.001
*
1
.0001
p
50
100
0
*
*
5
150
I
200
250
I
300
350
400
450
Temperature (oC)
Figure 5.15. 1(00)/1(450) vs. post-brush cure temperature for liquid crystal alignment cells
produced with Probimide 32 polyamide-imide alignment films.
219
(a)
Brush (left to right)
(b)
(c)
Figure 5.16. Depiction of the structure of the alignment layer films during alignment
relaxation process of the polyamide-imide. The amorphous chains begin randomly coiled
in the plane of the film (a). They are oriented by the brushing process (b). The orientation
is reversed by annealing above Tg (c).
220
In Chapter 4, it was found that a 3000 C curing treatment in air crosslinked the
polyamide-imide bulk films, increasing the glass transition temperature. To determine
whether this effect can be observed in surface alignment layers, the alignment relaxation
methodology was employed. Polyamide-imide alignment films were baked at 300 0 C in air
for 30 minutes. This treatment was expected to induce chemical crosslinks in the material
and increase its thermal stability. The films were then brushed to produce liquid crystal
alignment. Before building the alignment cells, the films were treated at an array of postbrush curing temperatures in order to see what temperature treatment destroyed the
alignment.
The alignment parameter 1(00)/1(450) is plotted against post-brush curing
temperature in Figure 5.17. The data in Figure 5.17 are compared to the data obtained with
no pre-brush 3000 C air treatment, i.e., the data in Figure 5.15. Clearly, the 3000 C air
treatment has increased the thermal stability of the films. The alignment material can
withstand higher temperature treatments before its aligning capacity is destroyed because
the material is chemically crosslinked prior to brushing.
IU
00
10
0o
O
0
.01
0
0o
0
.001
.0001
I
50
100
I
150
"'
I
200
I
250
I
300
350
400
450
Temperature (oC)
Figure 5.17. I(00)/I(450) vs. post-brush cure temperature for liquid crystal alignment cells
produced with brushed polyamide-imide that were either softbaked (filled circles) or cured
at 3000C in air (open circles) prior to brushing.
221
Poly(phenylene ether sulfone)
Poly(phenylene ether sulfone) was found to have strong liquid crystal aligning
properties. It was selected as a candidate for alignment based on its highly polar backbone
and its yellowish color. The chemical repeat unit of this polymer is shown below:
The bulk glass transition temperature is 2260 C [34]. The material was obtained as pellets
from Scientific Polymer Products and was dissolved to 3wt% solids in N-methyl
pyrrolidone. The material was spin coated to form films for liquid crystal alignment cells.
The alignment relaxation technique was employed for the poly(phenylene ether
sulfone) films. The relaxation "scan" obtained is shown in Figure 5.18. Destruction of the
alignment begins near 210 0 C and is complete near 230 0C, as frozen-in, oriented amorphous
chains are relaxed above Tg.
0
S
0
0
.0011
S
0
0
.0001
100
200
300
400
Temperature (oC)
Figure 5.18. 1(00)/1(450) vs. post-brush cure temperature for liquid crystal alignment cells
produced with brushed poly(phenylene ether sulfone) alignment films.
222
PMDA-APB Polvimide and the Effect of Imidization
The alignment relaxation of PMDA-APB polyimide films was investigated. The
polyamic acid precursor solution was obtained from the NASA Langley Research Center as
15wt% solids in dimethylacetamide (DMAc). The polymer solution was diluted in DMAc
to 5wt% solids and spin coated onto glass slides for liquid crystal cell construction. The
alignment film thicknesses were about 1500oA as measured by a Dektak 8000 profilometer.
The chemical repeat unit of the fully imidized polyimide is shown below. St. Clair [35]
previously reports a 2210 C glass transition temperature for this polymer.
The alignment relaxation technique was employed for the softbaked films and for
films baked at 2750C for 1 hour prior to brushing. The 1(00)/I(450) alignment parameter is
plotted against post-brush cure temperature in Figure 5.19. The relaxation temperature is
apparently quite low, approximately 170 0C, in the softbaked films, Figure 5.19(a), because
the material has not yet been converted from polyamic acid to polyimide. The glass
transition temperature is much higher in the 275 0 C pre-brush treated material, Figure
5.19(b). Tg is around 2500 C, which is only slightly higher than the previously reported
2210 C [35]. Clearly, the imidization reaction has increased the thermal stability of the
aligning properties of the material.
A series of PMDA-APB films were also pre-brush cured at 2210C for 30 minutes.
The alignment relaxation scan is shown in Figure 5.19(c) The thermal stability limit
appears to be below 250 0 C. However, there appears to be a reversal of the alignment
destruction slightly above this temperature. This reversal might be associated with the
completion of imidization of the material.
223
10
_____~_~_____
_____
_1_1_-~---·--·-----·^·----------
~I
__ _
__
_
__
a
1
S0O
oo
.1
0
.01
.001
.0001 I .001
II_ ,-100
lOO
3
200
300
400
Temperature (OC)
Figure 5.19. 1(00)/1(450) vs. post-brush cure temperature for liquid crystal alignment cells
produced with brushed PMDA-APB alignment films. a.) PMDA-APB poly(amic acid)
softbaked only.
224
b
0
•g
•
o
.01 i-o
0.
*0
**0
.001
.0001
a
•
0
0
I
100
100
aa
"
.
!
200
200
a0
I
(°C) 300
300
Temperature
Temperature (oC)
Figure 5.19. continued. b.) PMDA-APB hard baked 2750 C 1 hour.
400
400
225
10;
C
0
0
kn
.11
B
o
[]
.01
.001
0
•
0
"'
o
o
"
.0
13
3
.0001
0
100
200
300
Temperature (oC)
Figure 5.19. continued. c.) PMDA-APB cured at 221 0C for 30 minutes.
400
226
Poly(vinyl alcohol)
Poly(vinyl alcohol) has strong liquid crystal aligning properties. PVA is also
known to be a highly crystalline polymer. Based on that fact, it seems less likely that the
orientation of amorphous chains could be associated with alignment on PVA layers.
115,000 molecular weight PVA was dissolved at around 800 C as 3 wt% solids content in
water. The solution was spin coated onto glass as films for liquid crystal alignment cells.
The relaxation of the alignment is shown in Figure 5.20. There appears to be a
transition from aligned to unaligned which begins at around 200 0C and appears to be
complete at around 2250 C. This "thermoalignment" transition is consistent with the melting
temperature previously reported by Cebe [36]. Thus, it appears that for PVA, the
alignment is associated with the orientation of crystalline material. As the crystals melt, the
orientation introduced by the brushing process is destroyed. This data suggests that
alignment of liquid crystals on brushed polymer layers is a "phase-neutral" process in the
sense that it does not matter whether the polymeric film is crystalline or amorphous.
-M
m
1
0
.1
0
0
* 0
0
.001
0
.0001
I
I~
100
I
200
300
400
Temperature (OC)
Figure 5.20. 1(00)/1(450) vs. post-brush cure temperature for liquid crystal alignment cells
produced with poly(vinyl alcohol) alignment layers.
227
5.4. Experimental error
Experimental error was managed by several techniques, some of which have
already been discussed. First, systematic error due to twist distortions was minimized by
use of the adjustment technique described previously in Section 5.2.2. Error due to
variations in cell thickness from sample to sample was minimized by use of the alignment
parameter A=I(0 0 )/I(450 ) which should not be a function of cell thickness.
Still, statistical error can arise from variation in both the alignment strength and cell
thickness from spot to spot within the sample. This error was minimized by a two-fold
strategy. First, many data points were taken for each cell as previously explained in
Section 5.2.2. Second, many cells were constructed throughout the course of the research,
as is evident by the many data points in Chapters 5 and 6.
For I(00) and 1(450) measurements of individual cells, the standard deviations of the
means were on the order of 50% or less. In order to calculate the alignment parameter
A=I(00 )/I(450 ) for an individual cell, two mean intensities were divided. The propagated
standard deviation, Sw, for this mathematical manipulation of the data is given by [37]:
Sw2 = (1/y2) Sx2 +(x2/y 4) Sy 2
(6)
where x is I(00), y is 1(450), and Sx and Sy are the standard deviations in x and y
respectively. For 50% error in I(0') and 1(450), the propagated error is approximately 71%
for 1(00)/1(450).
The change in 1(00)/1(450) from a state of strong alignment to a state of no alignment
is approximately three orders of magnitude, as 1(00)/I(450) changes from about 10-3 to 100.
The error in a single data point is approximately 7% of that change. This clearly supports
the conclusion that the transmission method is, in a real and meaningful way, able to
distinguish aligned liquid crystals from unaligned liquid crystals.
By taking many data points, the uncertainty in the alignment measurements was
reduced. The alignment parameter A=I(00 )/I(450 ) was shown to be 1.04x10 -3 ± 2.8x10 -4
for brushed polyamide-imide aligned cells. The standard deviation in this measurement is
about 27% of A, and only 3% of the change in A that occurs during the transition from the
aligned to unaligned state.
To determine whether the measured transition from the aligned to unaligned state is
significant, a t-test is performed for the cells produced from softbaked polyamide-imide
alignment films (Figure 5.15). For aligned cells with post-brush cure temperatures 2650 C
and below, the average of the I(00)/1(450) values is 1.0x10- 3 ± 2.8x10 -4 . On the other
228
hand, for unaligned cells with post-brush cure temperatures 3000C and above, the average
of the 1(00)/1(450) values is 0.98 ± 0.20.
The quantity t can be calculated according to [37]:
t = (x - y) / [(Sx 2/Nx) + (Sy2/Ny)
1/2
(7)
where x and y are the two means compared, Sx and Sy are the standard deviations in x and
y respectively, and Nx and Ny are number of measurements in x and y. For comparing the
data sets of the aligned and unaligned specimens in Figure 5.13, the calculated value of t is
about 8.6. According to tabulated critical values of t [37], t would have to be greater than
2.45 in order to conclude that there is a significant difference between these two data sets
with 95% certainty, and above 5.96 in order to conclude that there is a significant
difference between these two data sets with 99.9% certainty. Thus, clearly, with t=8.6,
there is a significant difference between the aligned (post-brush cure <2650 C) and
unaligned (post-brush cure 2300 0C) data sets with greater than 99.9% certainty.
In conclusion, it is clear that the techniques described in this chapter provide a very
real and meaningful measure of the parallel alignment of the liquid crystals. Error in the
intensity measurements should not seriously affect the validity of the results.
5.5. Conclusions
Brushed layers of Probimide 32 polyamide-imide parallel-aligned E7 nematic liquid
crystals effectively. The main chains of the polyamide-imide are oriented during the
brushing process. Also, 450A films of the preimidized Probimide 32 resin appear to be
free of solvent after a 110 0 C cure.
Polyimide, poly(amic acid), poly(vinyl alcohol), and poly(phenylene ether sulfone)
also parallel-align E7 liquid crystals effectively. Empirically speaking, all of these
polymers are yellow in color and have highly polar backbones. Polycarbonate and
poly(methyl methacrylate) do not appear to produce parallel alignment of the liquid crystals.
The "relaxation of alignment layers" has been introduced as a technique for
evaluating thermal transitions in surface layers. When brushed polyamide-imide, PMDAAPB poly(amic acid), PMDA-APB polyimide, and poly(phenylene ether sulfone) layers are
treated above their glass transition temperatures, their ability to align liquid crystals is
irreversibly lost. This phenomenon has been taken advantage of in order to investigate
processing / structure / property relationships in polymer surface layers. In addition, it was
found that the ability of brushed layers of highly crystalline poly(vinyl alcohol) to align
229
liquid crystals was destroyed at its melting temperature, suggesting that polymer surface
alignment can involve the orientation of either amorphous or crystalline chains, depending
upon the polymer.
5.6. References
1. D. Berreman, Phys. Rev. Lett., 28(26), 1683 (1972).
2.
D. Berreman, Mol. Cryst. Liq. Cryst., 23, 215 (1973).
3. U. Wolff, W. Greubel, and H. Kruger, Mol. Cryst. Liq. Cryst., 23, 187 (1973).
4.
E. Lee, Y. Saito, and T. Uchida, Jpn. J. Appl. Phys., 32(12B), L1822 (1993).
5. Y. Zhu, L. Wang, Z. Lu, and Y. Wei, X. Chen, and J. Tang, Appl. Phys. Lett.,
65(1), 49 (1994).
6.
D. Seo, H. Matsuda, T. Oh-ide, and S. Kobayashi, Mol. Cryst. Liq. Cryst., 224, 13
(1993).
7.
D. Seo, T. Oh-ide, H. Matsuda, T. Isogami, K. Muroi, Y. Yabe, and S. Kobayashi,
Mol. Clyst. Liq. Clyst., 231, 95 (1993).
8. S. Ishihara, H. Wakemoto, K. Nakazima, Y. Matsuo, Liquid Crystals, 4(6), 669
(1989).
9.
E. Lee, P. Vetter, T. Miyashita, T. Uchida, M. Kano, M. Abe, and K. Sugawara,
Jpn. J. Appl. Phys., 32(10A), L1436 (1993).
10. Y. Kawata, K. Takatoh, M. Hasegawa, and M. Sakamoto, Liquid Crystals, 16(6),
1027 (1994).
11. H. Aoyama, Y. Yamazaki, N. Matsuura, H. Mada, and S. Kobayashi, Mol. Cryst.
Liq. Cryst., 72, 127 (1981).
12. N. van Aerle and J. Tol, Macromol., 27, 6520 (1994).
230
13. J.M. Geary, J.W. Goodby, A.R. Kmetz, and J.S. Patel, J. Appl. Phys., 62(10),
4100 (1987).
14. E. Lee, P. Vetter, T. Miyashita, and T. Uchida, Jpn. J. Appl. Phys., 32, L1339
(1993).
15. S. Kuniyasu, H. Fukuro, S. Maeda, K. Nakara, M. Nitta, N. Ozaki, and S.
Kobayashi, Jpn. J. Appl. Phys., 27(5), 827 (1988).
16. N. van Aerle, J. of the SID, 2(1), 41 (1994).
17. D. Seo, H. Matsuda, T. Oh-Ide, and S. Kobayashi, Mol. Cryst. Liq. Cryst., 224, 13
(1993).
18. N. van Arle, M. Barmentlo, and R. Hollering, J. Appl. Phys., 74(5), 3111 (1993).
19. T. Russell, MRS Bulletin, 21(1), 49 (1996).
20.
M. Ree, T.L. Nunes, and D.P. Kirby, Polym. Prep., 33(1), 309 (1992).
21. B. Factor, T. Russell, and M.F. Toney, Phys. Rev. Lett., 66(9), 1181 (1991).
22. H. Fukuro and S. Kobayashi, Mol. Cryst. Liq. Cryst, 163 157 (1988)..
23. Y. Nakajima, K. Saito, M. Murata, and M. Uekita, Mol. Cryst. Liq. Cryst, 237 111
(1993).
24. D. Seo and S. Kobayashi, Appl. Phys. Lett., 66(10), 1202 (1995).
25. D. Seo, S. Kobayashi, and M. Nishikawa, Appl. Phys. Lett., 61(20), 1202 (1992).
26. M. Nishikawa, K. Sano, T. Miyamoto, Y. Yokoyama, M. Bessho, D. Seo, Y. Imura,
and S. Kobayashi, Jpn. J. Appl. Phys., 33(7A), 4152 (1994).
231
27. S. Ishibashi, M. Hirayama, and T. Matsuura, Mol. Cryst. Liq. Cryst., 225, 99
(1993).
28. Amoco Ultradel 7100 series photosensitive polyimides, product information.
29. Personal communication, Dr. T. Gardner and Dr. R. Wenz, 3M Corporation.
30. J. Cognard, Mol. Cryst. Liq. Cryst., 78, Suppl. 1 (1982).
31. Merck E7 nematic liquid crystals, product information.
32. P.G. de Gennes, The Physics of Liquid Crystals, Ch. 3, Oxford Press, London,
(1975).
33. C. Gooch and H. Tarry, J. Phys. D: Appl. Phys., 8, 1575 (1975).
34. Scientific Polymer Products, Inc., product information guide.
35. T.L. St. Clair, in Polyimides, p. 58, D. Wilson, H.D. Stenzenberger, and P.M.
Hergenrother, Eds., Blackie, Glasgow (1990).
36. P. Cebe, Ph.D. Thesis, Cornell University (1984).
37. Laboratory Techniques Manual, Ch. 3, Department of Chemistry, Massachusetts
Institute of Technology (1979).
232
Chapter 6. Alignment of Liquid Crystals on Photo-irradiated
Polyamide-imide Layers.
6.1. Introduction
In the previous chapter, the alignment of nematic liquid crystals on brushed
polymer layers was discussed. As mentioned in Chapter 1, there is demand for techniques
of alignment which do not require the surface to undergo a contact treatment. Techniques
have been investigated to accomplish this, one of which is the exposure of a polymer layer
to linearly polarized light. In this chapter, it is shown that the Probimide 32 polyamideimide can be processed as a photo-induced surface alignment material.
Schadt et al. [1] previously showed that films of poly(vinyl methoxycinnamate)
(PVMC) exposed to linearly polarized ultraviolet light were effective as surface alignment
agents for nematic liquid crystals. The authors used a preferential crosslink argument to
explain the phenomenon. One serious disadvantage of using a cinnamate polymer for
alignment, however, is that it is a relatively obsolete photoresist material. It is not
produced today in large, commercial quantities, nor does it sell at low cost.
Hasegawa and Taira [2] recently showed that an aromatic/aliphatic polyimide
produced liquid crystal surface alignment upon exposure of the films to linearly polarized
short wave (X=257 nm) ultraviolet. This result was quite interesting because the polymer
was not specifically designed for photosensitivity. The authors used a depolymerization
argument, discussed in Chapter 1, to explain the alignment. One disadvantage of the
technique was that the alignment was attained only in a very narrow range of exposure
dosage, above and below which, no alignment was achieved. This could present
difficulties, for example, in reproducing the alignment scheme should any processing
conditions change.
In this chapter, a technique of alignment which uses the photo-irradiation of
polyamide-imide films is introduced. The photo-alignment process is compared to the
rubbing process. Information is also sought about the mechanism of alignment.
6.2. Experimental Section
6.2.1. The photo-alignment technique
The Probimide 32 polyamide-imide films and liquid crystal cells were produced as
described in the experimental section of the previous chapter, section 5.2.1. The brushing
233
alignment was replaced here, however, with the linearly polarized ultraviolet (LP-UV)
exposure process shown in Figure 6.1. The films were irradiated under a Spectroline
Model R-51A short wavelength (X=254 nm) ultraviolet lamp. The UV was passed through
an Ealing UV polarizer prior to incidence upon the bare alignment films. The films were
irradiated at a constant two inch distance from the lamp. The area-normalized exposure rate
was approximately lmW/cm 2. The alignment was evaluated by the qualitative approach
and laser optical transmission technique discussed in section 5.2.2.
UV Lamp
X= 254nm
I I I
J
i i i
Linear
to-Polarizer
t
I1
P
C
t
Coatedl
Slides
l
P
r
I
i
i
I~
E
,
Figure 6.1. Exposure technique for irradiating alignment films with linearly polarized
ultraviolet light (LP-UV).
234
6.2.2. Comments regarding the methodology by which PAI was selected
It would be appropriate here to make some brief remarks regarding the way in
which photo-alignment in the polyamide-imide was discovered. The Probimide 32
polyamide-imide was not selected as a candidate for photo-alignment studies with any
foreknowledge of its ability to align liquid crystals. In fact, many non-contact alignment
techniques were attempted before a successful technique was found. Producing liquid
crystal alignment by exposure of short wavelength LP-UV to virtually every alignment
material discussed in Chapter 5 was attempted. Exposure of inherently photosensitive
polyimide materials, discussed in Chapters 1 and 3, to LP-UV of i-line wavelength, which
crosslinks the material, was also tried. A non-contact, air jet brushing approach, and many
variations of it, were also attempted. However, the only technique which produced the
desired effect of inducing alignment was exposure of the polyamide-imide to 254 nm LPUV. In conclusion, the discovery of the photo-aligning properties in the Probimide 32
polyamide-imide was a culmination of an almost Edisonian style attack. Often, however,
this is the only effective way of attaining a desired result.
6.3. Results and Discussion
6.3.1. Evidence of liquid crystal alignment upon LP-UV exposure
Qualitatively, it was apparent that the LP-UV exposure to the polyamide-imide films
induced parallel alignment of the E7 liquid crystals. Cells produced from LP-UV exposed
polyamide-imide films exhibited a pattern of 900 extinctions consistent with an aligned
material, while cells produced from unexposed and unbrushed films were bright, i.e.,
exhibited no extinctions, when placed between crossed polarizers.
The alignment was therefore investigated by laser optical transmission. A liquid
crystal cell with the alignment films irradiated with LP-UV for 25 minutes was rotated
between the polarizer and analyzer with the alignment axis representing the 00 reference
axis. For a given rotation angle 0, the optical transmission I(4) for a perfectly aligned
system is expected to follow the equation:
I(0)/I(450) = sin 2 (24)
235
which is plotted in Figure 1.13 in Chapter 1. In Figure 6.2, the normalized optical
transmission I(4)/I(450) is plotted against 0 for the LP-UV aligned cell. The curve closely
approximates the theoretical curve from Figure 1.13, thus indicating the the LP-UV
irradiated polyamide-imide films effectively align the liquid crystals. The minimum
I(0)/1(450 ) at the 00, 900, and 1800 extinction angles, though greater than the theoretical
zero, is near 10- 3, which is comparable to brush aligned cells.
The alignment parameter A=I(0 0 )/I(450 ) is plotted in Figure 6.3 as a function of LPUV exposure dosage to the polyamide-imide layers. For very small dosages A is near
unity, indicating the absence of alignment. As exposure time increases, however, A
declines, indicating that the LP-UV treatment to the polyamide-imide is inducing alignment
in the liquid crystals. The alignment is very strong after about 25 minutes of exposure,
where I(00)/I(450) is comparable to the 1(00)/1(450) = 10- 3 obtained in Chapter 5 for brush-
aligned liquid crystal cells. The alignment remains strong for long exposure times. Hence,
it is shown that the window of exposure time required to produce alignment is quite wide.
.001
.0001
0
45
90
135
180
Angle (degrees)
Figure 6.2. I(0)/1(450 ) vs. rotation angle for liquid crystal alignment cell produced with
polyamide-imide alignment layers that have been irradiated with LP-UV for 25 minutes.
236
10"
.01
I
.001
.001
()
.0001
44
-
60
120
180
240
300
360
420
480
LP-UV Exposure Time (Minutes)
Figure 6.3. 1(00)/1(450) vs. LP-UV exposure time for liquid crystal cells produced with
LP-UV irradiated polyamide-imide alignment films.
237
6.3.2. Effects of unpolarized UV exposure
Fourier transform infrared spectroscopy
It was desired to understand how the ultraviolet irradiation affects the polyamideimide alignment film. FTIR spectra of unexposed and UV exposed polyamide-imide films
are shown in Figure 6.4. The region in which changes can clearly be seen is in the 12001900 cm- 1 range. The unexposed polyamide-imide film (Figure 6.4(a)) was irradiated with
unpolarized UV for one hour with an area-normalized exposure rate of approximate
7mW/cm 2 (Figure 6.4(b)). The most apparent change is the reduction in C=C phenyl
stretching near 1500 cm- 1 which declines about 50% in peak height relative to C=O
stretching at 1720 cm- 1 from the unexposed to exposed samples. Also, the N-C imide
vibration at 1370 cm- 1 declines in height about 25% relative to the 1720 cm- 1 peak. In
addition, the amide carbonyl band at 1680 cm- 1 recedes into a shoulder upon UV exposure.
These changes suggest that there are bond degradation reactions occurring in the polymer
upon exposure to UV with the phenyl rings the primary site.
Alignment layer relaxation
Upon exposing a polymeric material to extreme conditions, such as to high
temperatures or to irradiation, bond breaking, and possibly bond reformations, often occur.
The net effect on structure and properties will often depend upon whether crosslinking or
chain scission dominate the process. In Chapter 4, for example, it was found that upon
heating the bulk films to high temperatures in air, chemical changes lead to a net increase in
molecular weight. This was indicated by an increase in Tg and reinforcement of the
rubbery modulus that was apparent in the bulk dynamic mechanical spectra.
The effect of UV on the alignment films is difficult to measure, however, via
conventional bulk film measurements . For spin coated films 41tm thick, several hours of
UV lamp exposure has no apparent effect on the FTIR spectra of the material. Kambe [3]
reports that very long UV lamp exposure times, on the order of hundreds of hours, are
required to degrade the bulk mechanical properties of aromatic polyimides, polyamides,
and polyamide-imides. In any case, it may not be reasonable to assume that the effects of
long term exposure on bulk films are similar to the changes that occur in the surface layers
upon shorter exposures.
238
0
0
0
rM
E
.0
QD
T-
0
D
C14
(sl!un Aej~aqabe) eoueqjosqv
a
Iojela2p
Figure 6.4. Relative absorbance vs. wavenumber for a Probimide 32 polyamide-imide
film. a.) unexposed to UV, and
239
O
--
--
---------------------~~
C
C
-o
o
a
E
=
CP
o
C
C)
CI
C·1
-
~~~ -~---~I-~
(si!un AJe4l!qJe) eoueqJosqV eA!IleleL
Figure 6.4. continued. b.) exposed to UV.
0
240
Fortunately, a technique called alignment layer relaxation, through which the
thermal stability of surface layers can be investigated, was introduced in Chapter 5. This
method was used here to evaluate the changes in thermal stability of UV irradiated
polyamide-imide surfaces. Polyamide-imide alignment films on glass were exposed to
unpolarized UV for 2 hours. The films were brushed with rayon, and consequently cured
at an array of temperatures. Liquid crystal cells were built, and 1(00)/1(450) for the cells was
plotted against post-brush cure temperature. The results, shown in Figure 6.5, indicate that
the UV exposure reduces the Tg of the surface layers. This suggests that chain scission
reactions dominate upon exposure to short wavelength UV. This furthermore suggests that
the photo-aligning process in the polyamide-imide is associated with depolymerization
rather than with crosslinking.
a
.1
.001
.0001
-
-
I
100
I
150
S
200
I
250
I
300
II
350
400
Temperature (oC)
Figure 6.5 1(00)/1(450) vs. post-brush curing temperature for liquid crystal cells produced
with UV exposed (squares) and unexposed (circles) polyamide-imide alignment layers.
241
6.3.3. Effects of linearly polarized UV exposure
Fourier transform infrared spectroscopy
Linearly polarized UV and unpolarized UV appear to affect the polyamide-imide in
a similar way. Furthermore, dichroic FTIR spectra of LP-UV exposed material in Figure
6.5, in contrast with brushed layers, do not indicate the development of anisotropy. The
linearly polarized UV appears to reduce the height of the absorption at 1500 cm- 1 after 2
hours of LP-UV exposure (Figure 6.6(a,b)), and even more after 4 hours of LP-UV
exposure (Figure 6.6(c,d)) This change was also observed previously as a result of the
unpolarized UV treatment.
FTIR spectra taken with the incident IR axis polarized parallel to the LP-UV axis
(Figure 6.6(a,c)) appear to be identical to those taken with the incident IR perpendicular to
the LP-UV axis (Figure 6.6(b,d)). This is dissimilar to dichroic FTIR data for the brushed
polyamide-imide which indicated that the main chains were oriented in the direction of the
brushing. These data of the LP-UV exposed sample suggest that the linearly polarized UV,
unlike the brushing, does not physically move the polymer chains with respect to the LPUV axis. It is also possible that the dichroism produced is too small to detect by
conventional FTIR. This could be the case, for example, if orientation exists in only the
first monolayer. Because the mechanism of liquid crystal alignment on polymer surfaces is
not entirely understood, it is difficult to speculate as to what exactly is the mechanism of the
UV-induced surface alignment for the polyamide-imide.
Alignment layer relaxation
The alignment layer relaxation method was applied for liquid crystal cells aligned
with unbrushed, but LP-UV exposed, polyamide-imide layers. Many films were exposed
to 2 hours of LP-UV, a treatment which was shown previously (see Figure 6.3) to produce
strong liquid crystal alignment. The films were cured at an array of temperatures, and
liquid crystal cells were built. The value of I(00)/I(450) is plotted against post-UValignment curing temperature in Figure 6.7. It is apparent that the LP-UV exposed layers
do in fact show a thermal transition. This result indicates that, though ordering in the LPUV exposed alignment layers was not detected by FTIR, the relaxation of the film surface
reverses the liquid crystal aligning properties introduced by the LP-UV. It is also apparent
that chain scission caused by exposure to the LP-UV reduces the glass transition
temperature of the polyamide-imide surface layer.
242
o
o
E
o
°
O
o
E
>)
-o
C
1r
0r
(sl!un Ajeal!qje) eoueqJosqV eA!•eleH
Figure 6.6. Relative absorbance vs. wavenumber for polyamide-imide layers exposed to
linearly polarized ultraviolet (X=245 nm). a.) 2 hour exposure, UV axis parallel to incident
IR axis,
243
0
C)
0
0
o
C)
CN
0
C)
o
2
0
O
C)
2
=
E
C)
=
>
(sl!un •iedj!qje) eoueqjosqV a!jeledl
Figure 6.6. continued. b.) 2 hour exposure, UV axis perpendicular to incident IR axis,
244
0
0
0
0
SE0
a
r
o0
o
E
C)
0)
0
0
0
0o
(sl!un AjeJl!qje) aoueqJosqv a^!elae=
Figure 6.6. continued. c.) 4 hour exposure, UV axis parallel to incident IR axis,
245
0
0
0
0
0
CD~
CD
0
o
O
E
C.D
Lm
E0
E
6
(silun
Aaedqaed
) aoueqjosqV aiA!elae
Figure 6.6. continued. d.) 4 hour exposure, UV axis perpendicular to incident IR axis.
246
,,
A
A
1I
o001.0
.001 1
.0001
-
50
I
100
150
I
200
I
250
I
300
I
350
400
Temperature (oC)
Figure 6.7 I(0O)/I(450) vs. post-aligning cure temperature for liquid crystals cells produced
with polyamide-imide alignment films that have been exposed to LP-UV (triangles) or
brushed (circles).
Atomic Force Microscopy
An atomic force microscopy (AFM) image of a Probimide 32 polyamide-imide film
irradiated 2 hours with LP-UV is shown in Figure 6.8. There appear to be no microscopic
effects of the UV when compared to the AFM of pristine material shown in Chapter 5,
Figure 5.13(b). This is in contrast to the brushed films, shown in Figure 5.13(a), which
had a rough surface and grooves running in the direction of the brushing.
247
N
N
0, 0
0
U,
zC
0
N
Cq
w
Figure 6.8. Atomic force microscopy (AFM) image of LP-UV irradiated polyamide-imide
films.
248
UV stability of the alignment
The UV stability of the liquid crystal alignment was tested by the following
procedure:
1. Many polyamide-imide films were treated with an alignment inducing process, i.e.,
brushing or LP-UV exposure.
2. The films were then exposed to unpolarized UV, approximately 7mW/cm 2, for an array
of exposure times, at least two films per exposure time.
3. Liquid crystal cells were constructed.
4. 1(00)/1(450) was measured and plotted vs. post-aligning UV dosage.
Measured values of 1(00)/1(450) are plotted against post-aligning UV dosage in
Figure 6.9. The brushed films did not lose much of their aligning properties for any postbrush UV exposure. This is most likely because the alignment in the brushing alignment
process is associated with the physical movement of main chains in the alignment film.
Exposure to UV was apparently not sufficient to destroy the aligning characteristics of the
film.
For the LP-UV aligned layers, however, subsequent unpolarized UV exposure
appears to damage the ability of the polyamide-imide films to align the liquid crystals. This
experiment indicates that the LP-UV does not align the liquid crystals via the physical
movement of main chains. If that were the case, the unpolarized UV should not be able to
reverse the alignment, as it did not in the case of brushed films. Alternatively, if the LPUV exposure produces alignment by scissioning the polymer preferentially along one axis,
it is possible that further exposure of the material to unpolarized UV might act to deplete the
groups that are otherwise unaffected by the LP-UV, thus mitigating the aligning impact of
the LP-UV.
249
II
O
0
0O
.001 i
.0001 -
o
0e
0
I
i1
120
180
UV Exposure Time (Minutes)
Figure 6.9. 1(00)/1(450) vs. post-aligning unpolarized UV exposure time for liquid crystal
cells produced from polyamide-imide alignment films that were exposed to LP-UV (open
circles) or brushed (filled circles).
250
6.4. Conclusions
The parallel alignment of nematic liquid crystals has been accomplished by use of a
polyamide-imide alignment film that has been exposed to linearly polarized short
wavelength ultraviolet (X= 254 nm). Alignment was produced within minutes of exposure
and was maintained with very long exposures. Alignment layer relaxation and Fourier
transform infrared spectroscopy studies suggest that the short wavelength UV causes chain
scission in the polyamide-imide, though it is difficult to speculate at this time as to the
mechanism of alignment. Unlike the brushing technique, neither macroscopic grooves nor
bulk orientation of the main chains are produced in the polymer films by the LP-UV.
While brushing-induced alignment appears to be stable upon exposure to unpolarized short
wavelength UV, the UV-induced alignment appears to be damaged by subsequent exposure
of the bare alignment films to unpolarized short wavelength UV.
6.5. Subjects for further work
It was the goal of this research to demonstrate a new way of aligning liquid crystals
and to gain some better understanding into the science of liquid crystal alignment. Though
these goals were accomplished, this research has opened doors to topics for further
investigation. They are listed below, and are divided between the practical and scientific
questions. It will be quite interesting to see whether or not, over the next 5-10 years, these
questions are answered.
Can effective commercial devices be produced in an economical way via a photo-alignment
process? This raises many questions.
1. Can adequate surface pretilt be produced by a photo-aligning technique? And how
might one go about producing surface pretilt by a photo-aligning process? Might alkyl
side groups added to the repeat unit of the polymer or oblique irradiation of the
alignment film produce pretilt?
2. Can the speed of the photo-alignment process be improved such that it is comparable to
brushing? And how might one go about speeding it up? Will increasing the intensity
or adding a radical initiator work?
251
3. Can a commercial display with adequate switching speed and device lifetime be
manufactured via a photo-alignment technique?
4. If other polymers can be used in photo-alignment processes, how do they compare to
the polyamide-imide in terms of the alignment strength produced and the window of
exposure dosage required?
5. Will the photo-alignment work with other types of nematic liquid crystals or in other
liquid crystal technologies? Can it be used to align smectic or ferroelectric liquid
crystals? Can the photo-alignment approach be used in polymer dispersed liquid crystal
displays?
What is the mechanism of photo-alignment? This is probably a more difficult question
which probably also requires more insight into how the brushing-induced alignment
works. This raises the following questions:
1. What types of polymers produce liquid crystal alignment by the brushing process, and
which ones do not? Why?
2. What polymers have liquid crystal photo-aligning properties? What, then, are the
characteristics of these polymers?
3. How do liquid crystals interact with brushed polymer surfaces, and how does this
compare with the way they interact with LP-UV exposed polymer surfaces? Can this
be investigated by molecular modeling, second harmonic generation experiments, or
other surface sensitive techniques?
4. If brush-induced alignment has been suggested to be associated with crystallization, is
it possible that the UV produces surface crystallization?
5. Though the dichroic FTIR experiments could not be detect ansiotropy in LP-UV
exposed films, might there be other techniques that can? Is the orientation too small to
detect? Does it occur in only the first monolayer? Or might there be no dipolarity at all?
252
6.6. References
1.
M. Schadt, K. Schmitt, V. Kozinkov, and V. Chigrinov, Jpn. J. Appl. Phys.,
31(7), 2155 (1992).
2.
M. Hasegawa and Y. Taira, Rec. SID Int. Disp. Res. Conf., 213 (1994).
3.
H. Kambe, in Aspects of Degradationand Stabilization of Polymers, Ch. 8, H.
Jellinek, Ed., Elsevier, Amsterdam (1978).
253
Appendix A. Description of Nematic Alignment in Terms of the
Poincare Sphere
A.1. The Poincare sphere
The Poincare sphere [1,2] is a geometrical construction upon which any
polarization state can be described. Every latitudinal / longitudinal coordinate represents a
different polarization state. As shown in Figure A.1, the lines of latitude describe the
handedness and ellipticity (1) of the electric field vector. Light in the northern hemisphere
is left handed, while light in the southern hemisphere is right handed. The closer to the
equator, the lower is p. At the equator, 1 = 0, and the light is linearly polarized. At the
poles, 11 = 1, the north pole representing left circularly polarized light, and the south pole
representing right circularly polarized light. The equator is defined as 00 latitude, the south
pole +900 latitude, the north pole -900 latitude. The ellipticity P is related to latitude (I) by
equation (1).
I = 2 arctan p
(1)
-I
0=o
+1
Figure A. 1. Lines of latitude (I) on the Poincare sphere representing ellipticity ([3).
254
As shown in Figure A.2, the lines of longitude represent different azimuthal angles.
The prime meridian, 00 longitude, is the point where the azimuthal angle is 00, i.e., parallel
to the axis of reference. The azimuth (a) increases as we travel clockwise from 00
longitude, as described in equation (2), where L represents degrees longitude.
L=2 a
(2)
Thus, two points along the equator separated by 1800 longitude represent two linearly
crossed polarized states.
1800
Figure A.2. Lines of longitude (L) on the Poincare sphere representing azimuth (a).
A.2. Changes in polarization state on the Poincare sphere
As light passes through a substance, the state of polarization of the light can
change. Such a change can be represented by a relative positional change along the surface
of the Poincare sphere. If the light transverses multiple layers, the construction can be
repeated as many times as needed to determine the final polarization state. We begin at a
given point on the sphere. Movement on the sphere proceeds, in general, along an
appropriate circle on the surface of the sphere. (Equivalently, we can fix the initial point
and rotate the sphere along an appropriate axis.)
255
The polarization state of the light passing through the polarizer is quite easy to find
on the Poincare sphere. The emerging light is linearly polarized, and thus, rests on the
equator. For convenience, we can define this as zero azimuth, and place the initial state at
00 latitude and 00 longitude.
The change in polarization state as the light passes through a birefringent sample,
i.e., an aligned LC film, is found as shown in Figure A.3. First, we locate the point P
which represents the polarization state of the light before it enters the sample, that is,
linearly polarized light. Having chosen point P as our point of reference, movement along
the sphere proceeds as follows. We locate the point S which represents the fast vibration
axis of the retarding material. In the parallel aligned liquid crystal cell, this would be the
location on the equator which is perpendicular to the nematic director. Then, we draw the
small circle on the surface of the Poincare sphere which intersects P and is normal to OS,
the radial vector through S. The polarization state of the light after it has passed through
the sample is found by traveling from P clockwise (looking from S) along the circle an arc
length equal to 8 given in equation (24) of Chapter 1. We can, of course, produce the same
result by fixing the initial point in space and rotating the sphere about OS.
Figure A.3. Movement along the Poincare sphere from reference point P along the small
circle of the sphere centered on OS, where O is the center of the sphere and S represents the
azimuthal angle parallel to the fast vibration axis of the crystal.
256
As shown in Figure A.4, the change in polarization state as the light passes through
the analyzer is found by moving along the small circle on the surface of the sphere which is
in the plane normal to the pole-to-pole axis. In other words, we make purely a longitudinal
shift along a line of latitude. The arc length moved is twice the azimuthal angle of the
analyzer. We can, equivalently, fix the initial point and rotate the sphere about the polar
axis.
Figure A.4. The relative change in polarization state induced by the analyzer is a
longitudinal shift through the arc length between the analyzer (A) and the point of reference
(P). X1 moves to X2.
In Figure A.5, we suppose the position on the Poincare sphere to have moved from
P1l, position of the polarizer, to P2, after having passed through the sample, and then to
P3, after passing through the analyzer. The intensity of the light leaving the analyzer I/E 2
is shown by Jerrard [2] to be given by the simple expression in equation (2).
I/E 2 = cos 2 p
(2)
In equation (2), 2p is the arc length between the reference point (P1) and final point (P3).
257
Figure A.5. The arc length from reference state P1 to final state P3 is given by 2p. The
intensity transmitted by the system is cos 2p where full intensity is unity.
A.3. Examples of LC nematic orientation
Figure A.6 depicts the case in which the polarizer and analyzer are crossed, and the
liquid crystal plate is oriented with the nematic vector parallel to the polarizer. Points P, A,
and S represent the axes of the polarizer, analyzer, and fast vibration axis respectively.
There is no change in polarization state as the light passes through the sample because S is
diametrically opposed to P. Because the polarization state remains diametrically opposed to
A, position of the analyzer, no light is transmitted. In other words, the analyzer in azimuth
900 imposes a 1800 longitudinal shift. The arc length rtrepresents 2p, and by equation (2),
the intensity transmitted by the analyzer is zero, i.e. cos 2(n/2).
Figure A.7 depicts the case in which the nematic vector is 450 with respect to the
polarizer. While the circle in Figure A.7 represents the equator, the line though the middle
represents the prime meridian. As light from the polarizer is transmitted through the
sample, the polarization state of the light moves from the linearly polarized state at point P
to the elliptically polarized state at point E. In this geometry, light is permitted to pass with
the intensity given by equation (2). Note that a 450 sample orientation has the unique
property of imposing no change in the azimuth of the light ray.
258
S, A
Figure A.6. Parallel aligned LC plate with nematic vector parallel to P.
Figure A.7. LC cell with nematic vector 45' with respect to P and A. The light is
elliptically polarized by the sample as it moves along the prime meridian to point E.
259
A.4. Effect of misalignment of the liquid crystals
Figure A.8 demonstrates the effect of inhomogenous phases on the polarization
state of light as represented on the Poincare sphere. In Figure A.8, the alignment director
is parallel to the polarizer. In an ideally aligned system, the final polarization state will be a
single point A with reference to end point arc length 2p = 1800 (and cos2 p = 0). Unique
inhomogenous phases, on the other hand, will create a distribution of polarization states on
the surface of the sphere deviating from the single point A. Hence, the observed intensity
will be greater than cos 2 (900).
Figure A.8. Effect of random inhomogenous region on the Poincare sphere analysis. The
shaded region represents a distribution of polarization states deviating from A.
A.5. References
1. C. Viney, TransmittedPolarisedLight Microscopy, McRone Research Institute,
Chicago (1990).
2. H. Jerrard, J. Opt. Soc. Am., 44(8), 634 (1954).